To prevent generation of an error between the end face of a gear and a snap ring provided on the rotary shaft at a working stage by interposing an elastic member between the end face of the gear fixed to a rotary shaft relatively non-rotatably to the shaft and the snap ring provided on the shaft.

An annular groove 2b is punched to fit a belleville spring 9 to be a snap ring 8 and an elastic member adjacently to the end face of a gear 3 which is fixed to a rotary shaft 2 so that the gear 3 may be relative revolution impossible on the outer periphery of a rotary shat 2. The shaft line direction width and location of the annular groove 2b provided around the rotary shaft 2 are made the ones in which the belleville spring 9 is energized to the end face of the gear 3 by the snap ring 8 and is properly and elastically deformed. Thus, the gear 3 is prevented from moving in the shaft line direction to the rotary shaft 2 by constituting the belleville spring 9 supported by the snap ring 8 to energize the end face of the gear 3. An O-ring may be used as an elastic member in place of the belleville spring 9.