After graduating from University of Tokyo, he was a government official at the Ministry of Health and Welfare between 1986 and 1996. He became Associate Professor of Chiba University in 1996 and taught social security and public policy. He was a visiting scholar at the Department of Political Science of MIT between 2001 and 2002.
Professor Hiroi has written many books in Japanese including Social Security of Japan (1999), Steady-state Society (2001), Rethinking on Community (2009) etc. He received Japan Economist Award in 2000 and Osaragi Award in social sciences in 2010.
Since 2016, he is teaching at Kyoto university and is the vice director of Kokoro research center.