To resolve rattling in connection of a lower base with an upper base by setting a thickness of a circular flange to be smaller than a width of a circular engagement groove, and providing the circular flange with a protrusion to fill a gap to a circular engagement groove.

A circular engagement groove 17 is formed between an upper surface of a circular protruded part 13 in a lower base 11, and a lower surface of a high stage part 15 of a holder plate. A circular flange 21 extended from a recessed part 20 around a center hole of the upper base 18 is slidably engaghed with the circular engagement groove 17. A protrusion 22 is provided inside the circular flange 21 of the upper base 18, and a height of an upper surface of the protrusion 22 from a lower surface of the circular flange 21 is set to be almost similar to a width W of the circular engagement groove 17. The protrusion 22 thus fills a gap W-D1 between the circular flange 21 and the circular engagement groove 17 to prevent rattling between the lower base 11 and the upper base 18.