The Prime Minister of Japan (総理大臣) is the head of government of Japan. Since World War II, according to the Constitution, the Emperor appoints the Prime Minister among members of the Diet on the advice from the Diet. The office of Prime Minister has by convention been occupied by the leader of the majority party in the Diet, which has usually been the Liberal Democratic Party.
Because of the factionized and consensus nature of Japanese politics, the Prime Minister has very little power. His position as President of the majority party involves negotiation with party faction leaders, and legislation is usually initiated and reviewed by party committees rather than by the cabinet. Furthermore, substantial power is actually wielded by the Japanese civil service over which the Prime minister has little control.
See List of Japanese politicians for a list of Prime Ministers in Japan.