Dr. Miyadai was born in 1959 in Miyagi prefecture. He graduated from the Faculty of Letters, Tokyo University.
He got his Doctorate of Sociology from the University of Tokyo Graduate School of Sociology and was an Associate Professor at the University of Tokyo, College of Arts and Science from 1987. In 1990, he was a full-time lecturer at Tokyo University of Foreign Studies, and in 1993 became an Associate Professor at Tokyo Metropolitan University. He worked as an Associate Professor in the Tokyo Metropolitan University, Faculty of Urban Liberal Arts from 2005 and served as Professor from 2007 to March 2024.
While specializing in social system theory, he also became a leading expert in the research of subcultures, such as telekura (テレクラ), kogal (コギャル) and compensated dating (援助交際). He is also active on television and radio, becoming a popular figure on media.