If you follow the coverage in the western music press, it can be all too easy to come to the conclusion that Korea is the only country in Asia which has a thriving pop music scene. This is far from true. Japan had a highly developed and independent pop music culture many years before K-pop rose to predominance. This means that the music scene in the country is generally better developed and that there are more subgenres within Japanese pop music for fans to choose from.
Korean pop music has a (slightly unfair) reputation for only coming in one, bubble-gum laden form. Japanese music spans everything from heavy metal through to soft and syrupy ballads. Any band which you like from the west is very likely to have a Japanese equivalent. You’ll just need to do a little bit of research to find out who they are.
The best place to start is on a dedicated Japanese Pop Music page; here you’ll be able to see what is popular in the country and keep track of acts as they rise and fall in the charts.