It appears to have started to spread in the Kansai region (Kyoto-Osaka-Kobe) of W. Honshu. Authorities seem to be ready to take drastic measures with school closures. Some universities have posted notices saying that if even one case is confirmed, they will close temporarily.
See these Japan Times articles:
>>Monday, May 18, 2009
Domestic H1N1 flu cases increase to 42
Large outbreak hits schools in Osaka, Hyogo
By ERIC JOHNSTON
The number of domestic swine flu cases hit 42 on Sunday after 34 high school and college students as well as their family members and teachers in Osaka and Hyogo prefectures were confirmed to have been infected.
The confirmations followed the discovery Saturday of Japan's first eight domestic infections of the new H1N1 flu in Hyogo. A World Health Organization expert said community-level transmission may have begun in Japan, which could lead the WHO to raise its new flu pandemic alert to the highest level of 6 from the current 5.<< href="http://search.japantimes.co.jp/cgi-bin/nn20090517a3.html">http://search.japantimes.co.jp/cgi-bin/nn20090517a3.html
>>Kobe and Hyogo Prefecture officials announced nine steps they were taking to contain the spread of the virus and calm public fears. These included closing 75 public and private kindergartens, elementary, junior high and high schools, and universities in the city's Higashi-Nada, Nada, and Chuo wards, as well those in nearby Ashiya, until Friday. International schools in the designated wards, including the Canadian Academy, were also closed.<<