A bolt insertion hole is provided along an edge end portion on a flat surface of a plate-like member, and a tap hole is provided on an end face of the other edge end correspondingly to the bolt insertion hole. A bolt is inserted into the bolt insertion hole and screwed with the tap hole, and a box bearing is formed by means of four plate-like members joined together. The diameter of the bolt insertion hole is made larger than the diameter of the bolt. By changing the insertion position of the bolt with respect to the bolt insertion hole, the distance of a bearing gap inside the bearing, which is formed into the shape of a box, can be adjusted.