Bus

Hi there. There is no bus from the Yamanashi-shi station (JR Chuo Line) to Hottarakashi Onsen. You need to ride a car or taxi for 10-15 minutes from the station to the Onsen. It costs around 2,500 yen one-way.

By the way, here's the direction https://goo.gl/maps/DWb19rRPtjN2