First point of call would be the websites of record labels and the bands you are already interested in seeing. Tour dates are generally added as soon as they are confirmed.
I might be wrong, but the way your post is phrased it doesn’t sound like your priority is discovering Japanese music, just seeing bands you are already familiar with.
Do bear in mind that the costs of playing live in Japan are substantially higher than in Europe and entry prices to clubs and live music venues are probably steeper than what you will have paid in Europe (a typical tour in Japan will usually be just two cities - Tokyo and Osaka - in Europe, bands get to drive to more locations). Still, that expensive ticket price in Japan will probably include a drink or two.