The Allied occupation authority led by MacArthur was called SCAP, short for Supreme Commander for the Allied Powers.
SCAP referred both to the supreme commander and, commonly, to the headquarters and governmental machinery through which the occupation was administered. General Douglas MacArthur held the position from the beginning of the occupation in 1945 until 1951, when he was relieved during the Korean War.
SCAP supervised Japan’s demilitarization and helped implement extensive reforms. These included changes to the constitution, land ownership, labor organization, education, and the relationship between the military and civilian government. The occupation formally ended when the Treaty of San Francisco entered into force in 1952.
SCAP is sometimes confused with GHQ, because the headquarters was often called General Headquarters, Supreme Commander for the Allied Powers. SEAC, by contrast, referred to the Southeast Asia Command, a different Allied command during the war.