Ministry of Education, Culture, Sports, Science and Technology (MEXT)

文部科学省　全国の放射線モニタリングデータ

http://www.mext.go.jp/

MEXT

http://www.mext.go.jp/english/

Reading of environmental radioactivity level by prefecture, Time series data ( Graph ) ( English version)

http://www.mext.go.jp/english/radioactivity_level/detail/1303986.htm

Reading of environmental radioactivity level ( English version ) 

http://www.mext.go.jp/english/radioactivity_level/detail/1303962.htm