“DIET” is the name of Parliament of the country:

THE Diet is composed of the national legislature of Japan, and it is officially named the National Diet of Japan (国会, Kokkai). Additionally, the Diet is composed of the House of Representatives and the House of Councillors. Moreover, the word "Diet" has a historical link to German parliaments named after the Reichstag and even dates back to medieval Latin to mean an assembly, thus indicating various influences on Japan’s legislature.