Genpo was born the third son to Teiko MITSUKURI (Joan the third, a doctor of the Tsuyama Domain) in Nishishinmachi, Mimasaka Province (present-day, Nishishin Cho, Tsuyama City, Okayama Prefecture).
津山藩医箕作貞固（三代丈庵）の第三子として美作国西新町（現在の岡山県津山市西新町）に生まれる。