The general idea here is that 1) foreign crime has increased in the last ten years 2) that it has become more violent.
The thing is that the imigration law changed at the beginning of the 90' (92?) to make room for 'acceptable' foreigners as cheap labor, ie all the descendants of Japanese nationals who now live in south america for ex (there were massive emigration waves at the end of the 19th century, from here to S.A). Those second or third generation japanese have almost no trouble getting a visa, and they are the part of the foreign population that has grown the most since the new law has passed.
I did not check thouroughly but the impression I got from the figures I saw is that foreign crime has not progressed more than foreign population has.
Plus, most of the "foreign" crimes are related to visa/residence permit issues (I checked yesterday in the "white book on crime" published every year by the ministry of justice). I think the figure is about 1/3 of total arrest cases. I don't have the book at hand (I checked in a book store) but if people are interested in the figures I can find a way to reproduce them here.
from Asahi : Visa overstayers demonstrate in central Tokyo
http://www.asahi.com/english/asahi/0501/asahi050106.html
JC Helary