A motor-driven steering assist apparatus is disclosed having a single unit body covered by first to third housings. An upper end portion of an input shaft and a torque sensor are supported by the first housing. An upper end portion of an output shaft, an electric motor, a worm gear and a worm wheel are supported by the second housing. A lower end portion of the output shaft is supported to the third housing, and the second housing can be attached to the vehicle body side.