In a geared motor, a worm wheel and an output gear are rotatably supported about and slidably engaged with a center shaft. A transmitting plate connected to the output gear transmits rotation from the worm wheel to the output gear. An O-ring is fitted around the center shaft. The O-ring is sealingly surrounded on four sides in a seal retaining portion defined by an outer peripheral surface of the center shaft, an inner peripheral surface of the transmitting plate, an end surface of an annular protrusion of the worm wheel and one end surface of the output gear.