In the event that a moment in a bending direction is generated in a pinion shaft, for example, when a vehicle is involved in a collision or the like, since a bending facilitating portion which is a circumferential groove 111e bends, stress is concentrated hereon, and no large bending stress is generated in any case between a rolling bearing 102 and a support member 104. Consequently, an excessively large bending force is transmitted in no case to a four point contact type ball bearing 103.