Do I need to apply for a student visa?
U.S. citizens planning to stay just one semester or less in Japan do not need a student visa. Since the U.S. has a visa exemption agreement with Japan, U.S. citizens can stay in Japan for 90 days without a visa. (If you’re a non-U.S. student, check with your local Japanese Consulate about the need for a visa.)
If you plan to study longer than one term, or if you are not a U.S. or Canadian citizen, you need a visa.
For more, see Passport and Visa.