To always findthe next way forward


From a Leading
Jewelry Industry Firm

Kohsai factories are located
exclusively in Japan.
We harness our technological strength,
a fusion of craftsmanship and machining,
to maintain complete control over all
aspects of production: from quality,
to finishing, to flexibly responding to customer demand, and even to mass production.
We strive to improve the usability and reliability of our products as a technical jewelry company.


Kohsai is the first and only jewelry company in Japan to successfully commoditize forged rings.

A forged ring is manufactured by using pressure to stretch metal. The metal is then struck and forged.We are able to craft these with exceptionally high fidelity to target dimensions, with a margin of error of only 1%.We are the only jewelry firm in Japan to establish our own unique processing technology to deal in these forged rings and successfully commoditize them.These rings can retain detailed designs, thanks to the ability to apply uniform patterns to their entire circumference, as well as realize strong edge expression.

Kohsai's forged rings are the world's hardest and strongest

The uniquely potent hardness of our forged rings is thanks to special techniques employed in the forging process.It boasts class-leading strength alongside high elasticity and toughness against deformation.

Our manufacturing produces timeless pieces without imperfections often found in castings

Our processes completely eliminate air bubbles and rubber slippages usually found in castings.We aim to keep our products from shape and line degradation, so that our customers can enjoy them as long as they like.

So, an original jewelry brand by Isetan Mitsukoshi and Kohsai

So is a brand we jointly produce with Isetan Mitsukoshi, a renowned department store operator.

Kohsai brings its world-class techniques and original forging process to bear with Kohsai TANZOU®

Our forging process is unmatched, a delicately-layered fusion of advanced manufacturing and unique crafting techniques.The result is our Kohsai TANZOU rings, the world’s hardest and strongest.Kohsai TANZOU rings are characterized by accuracy and beauty in their form (a result of a balanced fusion of machine and craftsman), whilst being resistant to scratches thanks to their superior hardness.As proof, we have almost never had to remove a gemstone from a defective ring.Our forging process also allows resizing, offering excellent functionality versus others.