A center frame is fastened to a first bracket and a second bracket such that a first mated protrusion and a second mated protrusion are fitted together with a first mated recess portion and a second mated recess portion so as to be mated. O rings are disposed annularly so as to be held between an end surface of the first bracket and a first axial end surface of the center frame, and between an end surface of the second bracket and a second axial end surface of the center frame on an opposite side from outer circumferential inner wall surfaces of the first mated recess portion and the second mated recess portion.