From JR Tokyo Station, take the JR Shinkansen to Nagano.
From JR Nagano Station, take the Nagano Dentetsu train to Yudanaka. From Yudanaka, take a bus or taxi to Kanbayashi Onsen. From there, it's about a 30 min walk to the Jigokudani Yaen-koen entrance.
Get off the Joshinentsu Highway at Shinshu Nakano I.C. Follow Route 292. Follow signs that lead to Siga Heights. Drive the road to Kanbayashi onsen and park the car in the toll parking lot. It's about a 30 min walk to Jigokudani Yaen-koen entrance.