Consists of over 4,000 unpublished documents from the highest level of government, left by Kusuda Minoru, chief secretary to Sato Eisaku, the longest-serving prime minister (1964–1972) in postwar Japan. Dealing with a wide range of topics from diplomatic relations to domestic issues, they are essentially “Sato Eisaku documents,” including those classified as Top Secret.

The first phase, "Sato Eisaku Prime Minister's Office Documents," consists mainly of PM's Office materials from the Sato administration period, while the second phase consists mainly of supplementary documents from the first phase and subsequent documents from the Fukuda administration period to the Takeshita administration period.