To the best of my knowledge, Japan uses your regular twisted copper pair wires. They also have ISDN and cable of course, but they don’t/can’t provide ADSL internet services on those mediums** . Cable Internet is also available in Japan, and is probably ranked (a distant) 2nd insofar as popularity is concerned.
Personally, I am subscribed to the cheapest Yahoo! BB service (8Mbps). Unfortunately, living where I live, I am only getting about 1.2Mbps. If you were to go to www.dslreports.com and view the linespeed test results for people on the bb.yahoo.co.jp domain, you will see that people closer to Tokyo have a linespeed closer to the advertised speed, with the linespeed dropping the further you go.
**Actually, just recently, they are now offering ADSL on ISDN…not sure how it is actually done, but I have seen advertisements for it.