An electric power steering system includes a shaft coupling that drivably couples a worm gear to an output shaft such that the worm gear is tiltable with respect to the output shaft. The shaft coupling includes a fixing part coupled to the output shaft and a cylindrical part having an inner periphery in rectangular cross section and fixed to the fixing part. A coupling part that is inserted in the cylindrical part and that has an outer periphery in non-circular cross section, engageable with the inner periphery, is formed at a proximal end part of the worm gear, and the outer periphery is ridged such that gaps that allow the coupling part to tilt are provided inside the cylindrical part. Then, the cylindrical part presses an inner ring of the first rolling bearing toward an axially distal end side of the worm gear.