Up to present, very few noticeable embryological investigations on the general reproduction of Japanese bats have been made. Moreover, as the collection of the house bat, Pipistrellus tralatitius abramus (Temminck) is more difficult than that of bats living in the cave, the studies of this bat are few in Europe and America as well as in Japan. I have observed the following points in this bat. 1) It seems that in this bat the ovulation occurs on the last decade of April and the parturition takes place on the first ten days of July. So the period of gestation is considered to be about seventy days. 2) The number of the litter ranges one to three, the case of two youngs (fetuses) being most common. 3) Mother bat never fly holding her young.