Agreed, Adsense. Make some parking pages with content like DNF does but in Japanese:
https://www.google.com/support/adsense/bin/answer.py?answer=9727
"Google AdSense supports publishers in several different languages. Applications for participation in the program can be for sites with content primarily in:...Japanese...
....AdSense will serve relevant ads to your pages in the appropriate language, even if your site contains multiple supported languages."
Adsense has Geotargeting = Adwords advertisers can select which country or languge set they want their ads to be served to. If there is enough Japanese advertisers, it will work. If there is not, it will not work.