To achieve both securing of low temperature startability of an engine and reduction of rotational resistance of a balance shaft in a normal operation region.

On an outer peripheral surface of a balancer mass 23 of a balance shaft 2, a recess 23a extending along a rotation direction of the balancer mass 23 is provided. By providing the recess 23a at the balancer mass 23 this way, even when clearance Cmin between the outer peripheral surface (an outer diameter maximum part 23b) of the balancer mass 23 and an inner peripheral surface of an outer peripheral wall 12a of a housing 10 is made to be small, rotational resistance (flow resistance of gas and the like) of the balance shaft 2 can be reduced. Therefore, it becomes possible to achieve both securing of low temperature startability of an engine and reduction of rotational resistance of the balance shaft 2 in a normal operation region where the balancer mass is not immersed in oil.