http://en.wikipedia.org/wiki/Japanese_Instrument_of_Surrender#Text

We, acting by command of and in behalf of the Emperor of Japan, the Japanese Government and the Japanese Imperial General Headquarters, hereby accept the provisions set forth in the declaration issued by the heads of the Governments of the United States, China, and Great Britain on 26 July 1945 at Potsdam, and subsequently adhered to by the Union of Soviet Socialist Republics, which four powers are hereafter referred to as the Allied Powers.

this is signed by the emperor.

and potsdam said,
"Japanese sovereignty shall be limited to the islands of Honshu, Hokkaido, Kyushu, Shikoku and such minor islands as we determine," as had been announced in the Cairo Declaration in 1943."

http://en.wikipedia.org/wiki/Potsdam_Declaration