As of the start of 2018, Japan has no national qualification for medical interpreters. Some prefectures, cities and organizations have short courses and simple practical exams to train volunteer medical interpreters. There are also technical schools/private education organizations that have quite expensive medical interpreter courses. However, please be aware that NONE of these courses is a national qualification.
There are also many different medical interpreter associations but none of these can offer national a accreditation accepted by the Japanese government.
Please also be aware that it is very difficult to find paid work as a medical interpreter. Even large hospitals rely on volunteers. Hopefully this will change in the near future, but there is always the problem of who will pay the cost: should the medical facility or the patient bear the burden of cost?
If you would like to be a medical interpreter in Japan, please check with the city and prefecture where you live to find if they have any courses available. It is the best way to get a reasonable cost/free introduction to medical interpreting. If you decide that it is career for you, after you have some volunteer experience, try a private course but understand that right now it is impossible to receive a national qualification because one doesn’t exist.