A power window actuator including an electric motor, a reduction gear train for reducing rotational speed of the motor and a linkage mechanism connected to the gear train opens and closes a pivotable quarter window or a rear side vent. The rotation of the motor is mechanically stopped by structural members totally enclosed in a housing containing the motor and the gear train when the pivotable window comes to a fully opened or a fully closed position. An electric current supply to the motor is automatically discontinued by operation of a controller. An output shaft connected to a final stage gear of the speed reduction mechanism is extended outwardly from both sides of the housing. The linkage mechanism is composed of a pair of first links connected to the extended output shaft and a second link pivotably connected to the first links, so that the first links do not drop off from the output shaft without being fastened by a fastening member. Thus, the assembling process of the linkage mechanism is simplified.