Even though he was a great fan of Takaaki Yoshimoto, Miyadai was planning to become a natural scientist as a teenager.
In the 1990s, after characterizing the Yoga practices of Asahara Shoko of Aum Shinrikyo as expressing the inner core of early Buddhist asceticism, Yoshimoto was criticized along with Nakazawa Shin'ichi as a defender of Aum following the Sarin gas attack on the Tokyo subway.
As a disciple of Takaaki Yoshimoto, he wrote for Yoshimoto's magazine "Shikô (試行: Trial)".