An outer jacket externally fitted over an inner jacket so as to be slidable in an axial direction relative to the inner jacket has a slit extending straight from an opening formed at a jacket end. A recessed portion is formed at an inner edge of the slit such that a slit width at an inside diameter portion of the outer jacket is larger than a slit width at an outside diameter portion of the outer jacket. An amount of recess of the recessed portion in a circumferential direction of the outer jacket continuously decreases from the opening end to an area end.