Dear Clemente, I guess you are using CJK package for Japanese part. In that case, it's most likely that CJK package won't work just out of box. You need set up fonts manually. There is instruction for it in the following page (I'm assuming you can read Japanese):
http://oku.edu.mie-u.ac.jp/~okumura/texwiki/?LaTeX-CJK
If you are using pLaTeX, you are actually missing some required packages.
Koji
