To prevent a fly weight from running on a peripheral wall of a recess under an operation of an engine by receiving surely the fly weight with a stopper pin even when a thickness of a valve system cam gear is thinned to miniaturized the engine and even when a height of a peripheral wall face in the recess is made low.

The stopper pin 10 is provided in the cam gear 2, a tip part 10a of the stopper pin 10 is projected from an opening edge 7a of the recess 7, and the fly weight 3 oscillated by centrifugal force is received with the stopper pin 10 when an engine speed is raised, in the engine with a decompressor wherein the recess 7 is provided in one-side face of the cam gear 2 and wherein the fly weight 3 is arranged inside the recess 7.