Social Security System in Japan is quite good, if not perfect, as shown by indexes such as life expectancy and infant mortality rate. This lecture aims to get students to gain deep understandings on its characteristics, advantages and disadvantages, and lessons learnt from experiences of Japan, as well as its social backgrounds and circumstances, and also to provide useful and effective knowledge in order to establish/improve Social Security System in other countries. Each subsystem in Japan (Healthcare, Long-term Care, Pension, Labor Insurance, Public Assistance etc.) will be discussed. Explanation on reaction to COVID-19 will be limited, because it is an ongoing topic.(Amended on March 24 )