If Interac tries to pressure you into signing up for Kokumin Kenko Hoken, don’t do it! Kokumin Kenko Hoken is for people that are self-employed or unemployed. If you sign up for Kokumin Kenko Hoken, you may be forced to back enroll into the system up to the time that you started working in Japan (meaning you will have to pay your monthly dues up to the maximum limit of two years).
Instead, you should enroll into Shakai Hoken, because Interac will be forced to pay their half. If there is any back enrollment it will be covered by the company, not by you. You are all eligible for this. The only reason Interac tells you otherwise is because they don’t want to pay their portion of the money.
You can do this on your own, or you can join the “Interac union” (aka members of the Zenkoku Ippan Tokyo General Union Tozen ALTs) and we can force them to pay up together in solidarity. The Tokyo General Union has a lot of experience in forcing companies to enroll their employees into Shakai Hoken so we can get you enrolled with much less effort on you part.