Furano is quite a bit further than Rusutsu. There is no direct public transportation route that I am aware of. Public transit could be JR train, which in the best case is about 4 hours from JR Kutchan to JR Furano, requiring change of trains at Otaru, Sapporo, and/or Takikawa. The other option is to take one of the airport express buses back to Chitose Airport and then direct on to Furano. This is much more time consuming due to the connection time of the buses. The bus companies I am aware of which serve both Furano and Niseko are Hokkaido Access Network and Good Sports.
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