Kings-Evil is the most polished Japanese thrash band I've heard as of late. Very solid take on the genre. They only have one album, but they have been together since '89. Far and above the best Asian thrash band I've ever heard. Ritual Carnage, who was already mentioned are also worth a listen too. The Yamada brothers from Kings-Evil played in Ritual Carnage for a year when the band was on tour.
I know there are more good Asian bands and even a few more Japanse acts I've heard, but Kings-Evil was an easy notch above the rest IMHO. Their 2001 release is called "Deletion Of Humanoise".