Mochizuki was born on Ōsakikamijima, an island in the Seto Inland Sea, now part of Hiroshima Prefecture, where his father was an entrepreneur and ship owner.
Mochizuki lahir di Ōsakikamijima, sebuah pulau di Laut Pedalaman Seto, sekarang bagian dari Prefektur Hiroshima, dimana ayahnya menjadi seorang entreprenir dan pemilik kapal.