Take the JR Yamagata Shinkansen from Tokyo to Oishida Station (four stops north of Yamagata Station). The one way ride takes about 200 minutes and costs around 12,000 yen. From there, buses depart for Ginzan Onsen every 60-90 minutes. The bus ride takes about 35 minutes and costs 720 yen one way. The train portion of the journey is fully covered by the Japan Rail Pass, JR East Tohoku Area Pass and JR East South Hokkaido Pass, but the bus ride is not covered.