A gearing radius of a worm wheel is set corresponding to a neighborhood region of a steering neutral position, in such a way as to be relatively large. Thus, backlash is made to be relatively small. This prevents noises from being produced when an automobile travels straight on a bad road. Conversely, the backlash is set corresponding to most of regions of steering angles, which are within the remaining region thereof, in such a manner as to be relatively large. Thus, increase in resistance torque is suppressed. This is realized by a simple configuration in which a bias portion is provided in a part of the profile of teeth. In a case where power is transmitted from an electric motor by a worm shaft and a worm wheel during a steering operation, noises are hard to be produced even when the automobile travels on a bad road.