The purpose of this paper is to clarify the actual situation of segment financial information disclosure by Japanese companies. In Japan, segment financial information was institutionalized in the fiscal year ending March 31, 1991. First, the transition of this system was studied. Next, the author investigated the degree of segment disaggregation from 2000, to discuss how the information has been disclosed since institutionalization and the quality of the information. As a result, it was found that the state of announcement and the degree of disaggregation have improved since the management approach （MA） was introduced in 2011, however it became clear that many companies used the introduction of this MA as an opportunity to stop disclosing profit by region. Furthermore, after comparing the state of disclosure in Japan, the United States, Germany, France and the UK, it was found that business diversification is the most advanced in Japan. This means that segment financial information is more important in Japan than it is in major developed countries overseas.