PURPOSE: To prevent generation of the gear strike sound from a crank shaft balancer by forming it in a simple structure, with less number of component parts, and with less man-hours in assembly, by heightening the balance amount relative to the weight, and by setting small the starting torque.

CONSTITUTION: A cam 20 is formed at the tip of a shaft member 12 rotating together with a crank shaft 2, and the tip is fitted in the center hole of a weight 13, and a resilient member with a stopper is mounted in the center of the weight 13, and the cam 20 and resilient member are coupled together in such a way as capable of transmitting the torque and also absorbing excessive torque.