I'm from the United States, so I don't know how things may differ, but most undergraduate study-abroad programs are offered through your home university. I'm guessing yours does not offer them to Japan? If you're not certain, you should check. Even if they don't, you can probably set something up with them, though you'll have to do a lot of that work yourself I'm sure.
You should also search online for private study programs that aren't limited to a country. Waseda University in Tokyo
offers two summer programs through a university in the United States [though it allows participants from all over the world], as does
Sophia University. I don't know if that's exactly what you had in mind, but if it's to study the language it could be an ideal situation.
Also, with a school like Sophia you can probably get in contact with the administration easily and find out if you can simply enroll in one or two courses there, independent of your home school.
I hope that helps a little bit.