As shown in FIG. 30, each of the half housings 14A and 14B is a unitarily formed member which is made of, for example, a synthetic resin such as plastic or the like, and each of them comprises a sleeve shaped housing main section 14 b and a projecting flange 14 c which is provided as projecting to both sides from one end in the axial direction of this housing main section 14 b.