Records of heads of state
Officially, the current Emperor of Japan, Akihito is the 125th in line from the first emperor Jimmu, who is variously believed to have reigned in the 1st or 7th century BC. The earliest documentary evidence is only for the 29th emperor, Kinmei (AD 509–571); however, this is sufficient such that even the most conservative of estimates still places the Japanese imperial family as among the oldest lines in the world today.
Emperor of Japan
Unlike most constitutional monarchs, the Emperor is not even the "nominal" chief executive. explicitly vests executive power in the Cabinet, of which the Prime Minister is the leader. The Emperor is also not the commander-in-chief of the Japan Self-Defense Forces. The Japan Self-Defense Forces Act of 1954 also explicitly vests this role with the Prime Minister.