The aim of this paper is to show the overview of non-mainstream upper secondary schools. In Japan, non-mainstream upper secondary schools, such as part-time high schools, correspondence high schools, upper secondary specialized training schools, and support schools, take on a role of educational safety net for students who are not admitted to full-time high schools. This paper considers the following four questions to review papers about non-mainstream upper secondary schools. 1) What types of students tend to enter the non-mainstream schools? 2) What curriculums do non-mainstream schools organize? 3) What percentage of students drop out of non-mainstream schools? 4) What percentage of students graduate as part-time workers or the jobless?