To regulate a stress concentration on the tips of slits by setting a spring constant of its lengthwise directional intermediate part of beam parts formed in a spring main body by the mutual adjacent slits smaller than both end side parts.

In a flexure bearing 17, three slits 17d extending in parallel in a spiral shape are formed in a ring-shaped spring main body to an outer peripheral edge part from a central part. In beam parts 17e formed in the spring main body by the mutual adjacent slits, a width of an intermediate part 17e3 is set smaller than a width of an outside end part 17e1 and an inside end part 17e2 by enlarging one width among the mutual slits in a part where the respective slits 17d are adjacent in the radial direction so that a spring constant of its intermediate part 17e3 becomes smaller than the outside end part 17e1 and the inside end part 17e2. In detail, the respective beam parts 17e gradually become thin over the intermediate part 17e3 from the outside end part 17e1 and the inside end part 17e2.