To make an eccentric part of a crankshaft and the center lines of a balancer accurately coincide with each other by providing the balancer having a projection projecting in the radial direction of the central inside in a rotor part, and arranging a notch which corresponds to the highest part of the eccentric part and engages with the projection on the crankshaft.

A balancer 36 having a projection 36A projecting in the radial direction of the central inside is provided in a rotor part 31, and a notch 3B which corresponds to the highest part 2A of an eccentric part 2 and engages with the projection 36A is arranged on a crankshaft 3. That is, in the rotor part 31, the projection 36A is arranged in the radial direction of the central inside of the balancer 36, and the rotor part 31 and the crankshaft 3 are assembled so that the projection 36A of the balance 36 is engaged with the notch 3B of the crankshaft 3, and are fixed together by shrinkage fitting. Therefore, the eccentric part of the crankshaft and the respective center lines of the balancer can be made to extremely accurately coincide with each other, and a dynamic balance can be improved, and a noise can be sharply reduced.