To smoothly and speedily carry out assemblage only by adding a load simply in the axial direction by forming a serration head end part of a rotor shaft in a contracted shape in fitting structure of the rotor shaft of a motor and a ball housing on a 4WS actuator.

A head end part 14b of a serration 14a of a rotor shaft 14 is formed in a contracted shape to facilitate fitting. As the head end part 14b of the serration 14a of the rotor shaft 14 is formed in the contracted shape, it is possible to smoothly and speedily carry out fitting of the rotor shaft 14 and the ball housing 15 without requiring alignment of the serration 14a of the rotor shaft 14 and a serration 15a of the ball housing 15 only by adding a load simply in the axial direction. Consequently, assemblage of the 4WS actuator 5 is facilitated.