Between the lower endsurface 2b of the shaft member 2 and the inner bottom surface 7c1 of the bottom part 7c of the housing 7 is a thrust bearing part T. For ease of explanation, the side of the thrust bearing part T is taken as a lower side, and the opposite side as anupper side.The shaft member 2 is made, for example, of a metal material such as stainless steel.