A rotary direction regulating device for a synchronous electric motor. A plate is disposed on the rotor shaft which rotates with the rotor. A stopper is supported for axial movement in an eccentric axial hole through the plate. At opposite axial sides of the stopper, respective generally parallel, inclined cam surfaces move the stopper axially as the rotor rotates. The stopper is guided by the inclined surfaces to move axially and to permit the rotor and the regulating plate to rotate in the correct direction of rotation. One of the cam surfaces has a sharply inclined stopper wall oriented generally parallel to the axis of the rotor and the opposite cam surface shifts the stopper to abut the stopper wall upon reverse rotation of the rotor and the plate. That abutment prevents rotor reverse rotation. The stopper is supported for swinging with respect to the plate in its axial motion. The cooperating cam surfaces are opposite each other and are in cam plates disposed at opposite sides of the stopper.