There is provided a balance shaft housing (an upper housing 14U, a lower housing 14L) disposed in an oil pan (7) for accommodating balance shafts (13L, 13R) provided with counterweights (19) for canceling out a vibromotive force caused by pistons, characterized in that the housing comprises housings which are divided along a plane which is parallel with the balance shafts, and gaps (37) are formed between edge surfaces of respective side walls of the divided housings which are brought into contact with the plane for discharging lubricating oil from the housing, and that eaves-like projecting portions (38) are integrally formed on the upper housing (14U) of the divided housings for covering the surfaces confronting the gaps, whereby the eaves-like projecting portions help not only prevent lubricating oil from flowing into the housing but also enhance the rigidity of the housing.