-Capsule, a unit made up of 2 members: Yasutaka Nakata (producer and composer) and Toshiko Koshijima (the singer). Nakata also has his own record label and produces many other electronic music artists, like MEG, COLTEMONIKHA...
He's also the producer of the very catchy electronic female group Perfume, that's huge in Japan recently.
Some other artists:
Fantastic Plastic Machine
PLUS-TECH SQUEEZE BOX
Also Sakanaction (ﾆ探ﾆ谷ﾆ段ﾆ誰ﾆ歎ﾆ停?｡ﾆ停??, although they're a rock band, but they are very influenced by electronic music.