what is the role of the emperor there then?
His role is ceremonial and he has no powers related to Government.
Government of Japan
 His role is ceremonial and he has no powers related to Government. 
what is the type of government in japan It is the central government of Japan is the country ruled by the emperor or the prime minister? The Prime Minister
wiki:20866984
Government of Japan [SEP] Introduction
Instead, it is the Cabinet, comprising the Ministers of State and the Prime Minister, that directs and controls the Government and the civil service. The Cabinet has the executive power and is formed by the Prime Minister, who is the Head of Government. He or she is designated by the National Diet and appointed to office by the Emperor. The National Diet is the legislature, the organ of the Legislative branch. It is bicameral, consisting of two houses with the House of Councilors being the upper house, and the House of Representatives being the lower house. Its members are directly elected by the people, who are the source of sovereignty.