A curvature radius of a raceway groove of an inner ring of a first bearing supporting a first end portion of a worm shaft and a curvature radius of a raceway groove of an outer ring of the first bearing are larger than 50% of a diameter of each rolling element of the first bearing, whereby the worm shaft is pivotable about the first end portion. Internal clearances in the first bearing are eliminated because the first bearing is pressed in a radial direction by a pressing member. When the worm shaft vibrates relative to a housing in an axial direction, paired second elastic members and damp and absorb the vibration.