PURPOSE: To prevent the wrong insertion of an arm shaft into a thickness- stealing groove in regard to a mechanism which possesses a resin gear having a groove cam, and an arm having a shaft to be engaged with the cam groove, and in which the arm is swung by the rotation of the cam gear.

CONSTITUTION: The thickness of a break-up relief groove to be provided at a cam gear 1 is made narrower than the diameter of a pin 3, and especially at a part where large thickness-stealing is necessary for break-up relief, plural thickness-stealing grooves 1b are provided. At a result, at the time of assembly, an arm 2 and the cam gear 1 can not be engaged with each other and rising to the surface is brought about, so the effect of the early detection of wrong work can be realized.