Mikio Aoki was a Japanese politician who served as the Chief Cabinet Secretary from 1999 to 2000, and was briefly acting prime minister following Keizō Obuchi's coma. A member of the Liberal Democratic Party (LDP), he also served as the Chairman of the LDP in the House of Councillors.

The chief cabinet secretary of Japan is a member of the cabinet and is the leader and chief executive of the Cabinet Secretariat of Japan. The chief cabinet secretary coordinates the policies of ministries and agencies in the executive branch, and also serves as the government's press secretary. The secretary is a statutory member of the National Security Council, and is appointed by the emperor upon the nomination by the prime minister. The chief cabinet secretary is the first in line of succession to the prime minister, unless the office of the deputy prime minister is occupied.