A two-roomed and overlapping box was designed and made. It is not only easy, but also practical to use it to propagate Microtus fortis. Fu rthermore, while comparing with the ordinary box, the pregnant rate, the propaga tional rate, the survival rate of their progenies and the propagational-survival rate of M. Fortis has been increased by 20%, 33.3%, 51.0% and 100.1%, respectiv ely.