The Japanese music scene has everything to offer that any other country has to offer. Most people don't get further then listening to Morning Musume, Ayumi Hamasaki or Gackt, but if you put some time in it you'll be amazed.
Alternative is a broad term, but i suggest, for starters, you listen to Bonnie Pink, Quruli, Antennasia, Sketch Show, Cornelius, The Back Horn, Shiina Ringo, The Brilliant Green, Takkyu Ishino, Fake?, Buck Tick, The Elephant Kashimashi, Blankey Jet City, The Mad Capsule Markets, Electric Eel Shock, Thee Michelle Gun Elephant, The Boom Boom Satellites, Fake?, Rize, Sads (or Kiyoharu solo), Yasuyuki Okamura, Soft Ballet...
Like i said... for starters. Have fun discovering.
