A once very popular climbing area that was closed due to the impact on the tourist scene. Saying this, the Arakawa has created an amazing bouldering area. Spread over two miles are some of Japans largest and most beautiful boulders. The granite boulders offer a very nice secluded experience. On busy days you will run into maybe two or three people.
There are dams on this river that regulate the flow of the river. Keep this in mind especially when you are bouldering near the water. There are sirens that ring and then a Japanese person announces what the increase in flow will be. In short if you are bouldering near the water and the sirens sound, move to the boulders higher up. There are many many options so do not be discouraged. This also means that sometimes your project will be dry and the next day be in two feet of water.
From Tokyo take the Chou Expressway to Kofu. Take the hwy 20 exit. Go west until you reach 358. 358 meanders through central Kofu, but eventually turns into 104 via a couple of unnamed streets. This will take you into the Vally. Once on 104 there is a split right for hwy 7, take this. Take this road until you see amazing rock formations and great bouldering. From 7 you can take 112 farther up the river to even more boulders.
I suggest to use Google Maps before you head to this area to become more familiar with these directions. It is also wise to purchase a Japan road atlas.