Kagura, northern Honshuu
Last winter I went to a place called Kagura ski resort, located on Mt. Naeba. This area is in Niigata which is on the northern part of the main island in Japan; about a 3 hour drive from Tokyo. You can always take a bus in the early morning from Shinjuku on a snowboard day trip, but we took a car.
This was a nice mountain with great snow, and a variety of slopes for both beginners and intermediate. I wouldn't call it advanced. I am more of an advanced rider, but it's always fun to snowboard in a group and teach people when you can.
Japan has a wealth of ski spots, and this is just one of them. You can check out more on Kagura and other spots in Japan here: