Watch writer Shun Horiuchi reviews the "No. 5 Kai," the first satellite hour watch made in Japan by Otsuka Low-Tech, released in 2025. Having previously owned the "No. 7.5" and "No. 6" models from the same brand, the writer goes beyond just the feel of the watch and comments on the value it achieves through its synergy.
Photographs & Text by Shun Horiuchi
[Article published on January 17, 2025]
A review of the Otsuka Low-Tech "No. 5 Kai"
Anyone familiar with the "7.5" and "6" models will undoubtedly recognize the "5" model as an Otsuka Low-Tech watch, and it's one that more clearly showcases the watch's unique worldview. Its design makes the most of the texture of the metal itself, primarily stainless steel, its unique kanji and katakana fonts engraved and inked, and its complex time display and gimmicks. The fact that this watch was born in Japan and is currently sold exclusively in Japan is the envy of watch enthusiasts worldwide. The fact that examples of this watch are sold at overseas auctions for several times the retail price is a good indication of its scarcity.
Until now, Otsuka Low-Tech has collaborated with the watch company Tokyo Watch Precision to release the No. 7.5 and No. 6 watches. While there were some design adjustments, they were faithful to the originals, compared to the watches produced solely by the brand's founder, Jiro Katayama. However, the No. 5 Kai released this year is unmistakably the No. 5 released in 2012, and while it respects the original as before, it is a completely redesigned watch with a drastic change in the time display gimmick. Why did it end up like this? It seems to be due to the significant "synergy" that has emerged in watchmaking with Tokyo Watch Precision. I would like to share my own thoughts on the matter below.
The synergy created by Otsuka Lowtec and Tokyo Tokei Seimitsu can be summed up in one word: "We can do more."
The synergy created by Otsuka Lowtec and Tokyo Tokei Seimitsu can be summed up in one word as "the ability to do more."
This satellite hour watch combines a moving hour disc like a satellite with a fixed scale to tell the time. Unlike the previously produced No. 5, this new model, dubbed "Kai," is the first Japanese-made wristwatch to feature this satellite hour function. The case is 12.2mm thick including the crystal. Automatic movement (MIYOTA 90S5 + in-house satellite hour module). 25 jewels + 2 ball bearings. 28,800 vph. Power reserve approximately 40 hours. Stainless steel case (40.5mm diameter, 7.6mm thick). Water resistant for everyday use. Price: 748,000 yen (tax included).
For example, to make the No. 5 Kai design work as a wristwatch, the first thing required is a towering, box-shaped sapphire crystal. Such a crystal is a part exclusive to this model, and unless a certain MOQ (Minimum Order Quantity) is expected, it cannot be obtained at a reasonable price. In other words, a stable production system with a considerable capacity is essential. As mentioned in our impressions of the No. 7.5, it uses a special lens-shaped crystal that appears to be exclusive to this model. The fact that we have been able to establish a stable production system that allows for the use of such special parts is a major synergy.
There's another synergy worth mentioning. The No. 5 Kai was announced on January 15th of this year at a joint press conference with MinebeaMitsumi, a manufacturer of bearings, motors, and other electronic components. HAJIME ASAOKA, a brand created by Hajime Asaoka, president of Tokyo Tokei Seimitsu, has traditionally used extremely small ball bearings in its watches, highlighting the relationship the company has built. It was only thanks to this collaboration with Tokyo Tokei Seimitsu that the guide roller that rotates the hour disc was able to incorporate a newly designed ball bearing with an outer diameter of just 2.5mm, an inner diameter of 1.0mm, and a width of 0.8mm.
It is certain that these "challenges to realizing ideas" are resolved through the relationship between the two parties, and we watch enthusiasts are benefiting from this synergy.
Jiro Katayama's trial and error and its results
To create a highly original and high-quality watch like the No. 5 Kai, a high-level design is essential, with Katayama going through various prototyping processes to solve problems and incorporate his ideas into the finished product. One example is the brass reduction gear located under the hour disc at 9 o'clock. Apparently, backlash in the satellite hours was an issue with normal gears, so after trial and error, they decided to create a reduction gear with two gears, one above the other, that are out of phase with each other, thereby avoiding backlash.
This is a bit of a detailed discussion, but this time display mechanism is what's known as the Vagabond Hour mechanism or Satellite Hour mechanism, which was first used in a Japanese-made watch, and famous examples include Audemars Piguet's "Starwheel" and Urwerk's "UR-100." Because the display mechanism is unique, it looks nothing like a regular two- or three-hand wristwatch, and this No. 5 Kai is also extremely distinctive and original, with the mechanism exposed to the front and covered by a box-shaped sapphire crystal crystal.
The base movement is a Miyota automatic, as before, and unlike the No. 6 and No. 7.5 models, it does not have a see-through case back, so the rotor and other components are not visible. Conversely, it could be said that the design allows you to see everything that needs to be shown from the surface. Let's take a look at the details.
A mass of precision metal parts covered by a sapphire crystal
In a typical watch review, the watch would be broken down into parts such as the dial, hands, case, and crown, and each would be written in detail. However, this watch has many visible parts that make up the satellite hour display, so it is not possible to write in a uniform manner.
The dial itself is the fan-shaped minute scale at 3 o'clock, a precisely machined part made of nickel silver. The next thing that catches your eye is the central part of the dial that suspends the three hour discs. Its surface is radially textured, and flathead screws are fastened to both ends of the milled edges. Another thing that catches your eye are the three regulation springs around the outer periphery of this part. Regulation springs, which are often found in calendar and chronograph movements, are usually invisible, but in this watch they are mounted on the top row, giving them a strong presence. Their slender finish also effectively accentuates the delicacy of this watch. Not only are these triangular parts visible, but the machined tool marks that can be seen on the surface of almost every component are a distinctive feature and appeal of Otsuka Low-Tech watches.
While deburring and other finishing processes are naturally carried out, the Otsuka Low-Tech brand is unique in that it does not entirely adopt the shiny, mirror-like hand finish that is the norm for high-end mechanical watches, and this is also one of the reasons for its reasonable prices. It's so mechanical that it seems as if almost all of the parts were made with end mills. I can picture the moment when they are removed from the chuck while covered in cutting oil and still hot, and the "workpiece" is turned into a "part" through machining.
Of course, that doesn't mean that there's no hand-finishing involved. The chimney-shaped bezel to the top of the lugs are likely machined, but the hairline on the side is a straight vertical line, so it appears that a hairline finish was added later to match the aforementioned machined lines. The rounded ends of the lugs on the strap side were also later hand-finished. The complete lack of incongruity in these areas is truly impressive, and it's clear that this watch was produced by a master craftsman with a deep understanding of metalworking.
One level above the movement is a plate engraved with "Otsuka Low-Tech Tokyo" in the "famous font." This has a sandblasted finish. The red arrow starting from the 12 o'clock position is a nice accent. The fan-shaped "dial" at 3 o'clock is also engraved with numbers using the "famous font" along with delicate tool marks.
The layer below that, visible at 5 o'clock, is circular-grained, creating a striking contrast with the sandblasted surface of the plate mentioned above. This is silver-colored because it is stainless steel, but the circular-grained surface visible at 12 o'clock is brass gold. The modular gear below the hour disc, which is relatively large for a mechanical watch, is also made of brass. This silver base with some parts in brass gold, various fonts in black ink, and arrows in red, creates a stunning use of key colors. The metal materials were likely chosen for the right place, but this configuration reminds us that function is also beauty.
Furthermore, the MinebeaMitsumi ball bearings blend seamlessly into the No. 5 Kai's components, giving it a unique, slightly steampunk feel that is a testament to Katayama's unique sense. It's the ultimate "mechanical watch made by a machine" among "mechanical watches." That said, legibility isn't sacrificed, and despite the unusual time display, it's easy to use.
About the gimmick
The actual movement of this watch appears to be limited to the part that suspends the hour disc rotating once every 12 hours and the second disc at 6 o'clock, which serves as a permanent second hand, rotating modestly. However, if you look closely, you can see the gear in the center of the dial that drives the second disc, and next to the second disc, you can see the balance wheel swinging at a fairly deep level, giving the watch as a whole a sense of movement within its stillness. Miyota's Cal. 90S5 movement is a center-seconds watch, so a gear train has been added to bring the second disc drive mechanism to the 4 o'clock position, and this gear can be seen when viewed from the 3 o'clock position. This drives the second disc counterclockwise.
Additionally, one of the highlights of the No. 5 Kai is the 90-degree rotation of the hour disc as it passes through the ball bearing fixed at 8 o'clock. However, since a single hourly "movement" might seem lacking, the presence of the seconds disc and the glimpsed balance wheel is significant. Their placement is also exquisite, as they are placed on a lower level without interfering with the main hour disc. Yes, this watch has a multi-layered structure, and its distinctive feature is that almost all of it can be seen through the huge box-shaped sapphire crystal glass. Therefore, the movement itself is not the main focus, but is located on the lowest level, and as mentioned above, it does not have a see-through caseback.
So how does it feel to use?
The lug width is 22mm, and it comes standard with the famous two-rib calfskin strap, making this watch a true Otsuka Low-Tech. However, the strap has the smallest diameter holes, so even when fastened with the buckle, it was a little too long for my slender wrist, so I switched to a strap of my own for my impressions.
The solid case back is quite flat, with the lugs pointing slightly downward, giving the impression that the watch sits perfectly flat on the wrist. With a see-through case back, the case back would probably be about 1mm thicker, allowing you to fully appreciate the benefits of a solid case back.
With a solid case back and the movement at the bottom, the winding stem is located at 4 o'clock on the side of the case, close to the wrist. Naturally, this would affect the diameter of the crown, but the size is just as good as a regular crown, and the crown doesn't touch the wrist, which is annoying. As with other Otsuka Low-Tech models, the sharp knurling makes the crown easy to turn, allowing for delicate time adjustments.
Additionally, the sapphire crystal glass that rises from the bezel allows the watch to slide out smoothly from the cuff of a shirt without getting caught on it. The time display is completed in the halfway point at 3 o'clock on the watch, so it is actually easy to read as long as it is worn on the left hand. Because of this gimmick, I initially thought it might take some getting used to to tell the time, but legibility is not sacrificed, and in fact, once you get used to it, it actually feels quite easy to read. My impression of legibility among Otsuka Low-Tech watches is that "Size 5 modified ≒ Size 6 > Size 7.5."
The second disc has a scale, so the accuracy of the No. 5 Kai can be confirmed. When this particular watch was used continuously for a week, the total accumulated value was +23 seconds, which is excellent.
It is a watch that is perfectly practical and comfortable to wear, making it suitable for daily use.
Finally
As of May 2025, when this article was written, Otsuka Low-Tech watches are still sold only in Japan by lottery. Whenever they appear at overseas auctions, they still fetch far more than expected, demonstrating their global desirability. Seeing and using one in person makes it easy to see why. As they are not limited editions, they will likely continue to be produced, but I hope that any lucky enthusiasts who manage to get their hands on one will use it to the fullest without reselling it.
Katayama's future creations also appear to include watches in a higher price range. These may become more out of reach, but we are already looking forward to seeing what kind of models he will come up with. We will be keeping a close eye on Katayama's future activities.
