After retirement, Nagai became an assistant coach for Furukawa Electric (later "JEF United Ichihara") in 1988. In 1989, he became a manager for Japan U-20 national team. In 1992, he returned to JEF United Ichihara and managed until 1993. In 1996, he signed with Japan Football League (JFL) club Fukushima FC and managed in 1 season. In 1998, he signed with JFL club Albirex Niigata. The club joined new league J2 League in 1999. In 2001, he moved to Yokohama FC. In 2006, he signed with his local Nadeshiko League club Urawa Reds Ladies.