There are more than 260 Catholic churches in Japan. You can find the nearest one here, at the website of the Catholic Bishops Conference of Japan.
If you plan to attend Mass in Japanese you might it useful to download or print this PDF. It’s a scan of a booklet that helps you follow and participate in the liturgy. The booklet is available at the door in some Japanese churches, but many don’t have it, so take along a copy.
You may be surprised to find that Catholics in Japan do not genuflect to the Tabernacle but bow. And during Mass they stand at times when others might kneel.