The National Capital Region as defined by the 1956 National Capital Region Planning Act
The National Capital Region(首都圏,Shutoken?) of Japan refers to the Greater Tokyo Area as defined by the National Capital Region Planning Act (首都圏整備法,Shutoken-seibi-hō?) of 1956, which defines it as "Tokyo and its surrounding area declared by government ordinance." The government ordinance defined it as Tokyo and all six prefectures in the Kantō region plus Yamanashi Prefecture. The population was 42.92 million as of (Oct 2009 using annual statistical office estimates).
Note that while this all inclusive term includes all of Greater Tokyo, it also includes sparsely populated mountain areas as well as far flung Ogasawara islands which are administered under Tokyo.