A reverse rotation preventing mechanism for synchronous motors includes a stopper interposed between a frame of a synchronous motor and a suitable stationary member, the stopper, including a stopping portion having elasticity and a rotating member fixed to a rotor shaft of the synchronous motor, and the rotating member, including a metallic flywheel and a gear disposed on the central portion of the flywheel, the flywheel having an engagement portion. The engagement portion of the flywheel of the rotating member is engaged with the stopping portion of the stopper when the synchronous motor is rotated in the direction opposite to a predetermined direction.