Otoineppu Trail Run
50k, 30k, and 15k events.
The event is run at the Otoifuji Ski slope and over cross country ski courses. From the course map, it looks fairly challenging, with a lot of climbing to be done.
They have a Facebook Page here: https://www.facebook.com/2013.trailrunning.otoineppu
And the event page on the Otoineppu Homepage, where you can download course maps, is here: https://www.vill.otoineppu.hokkaido.jp/kakuka/chiikishinkou/oshirase/2013-0702-0939-16.html
Too late to apply since the event is this Sunday, the same day as the Hokkaido Marathon, but this may be one to put on the schedule for next year.