To provide a locking mechanism for a bolt which can be tightened easily, regardless of where the bolt is located.

In a mounting space 14, an inner side 18 of a recess 15 set up a projection 19 separating from the bottom 16 of the recess 15. Each projection 19 is tapered 19d from an upper part 19c to the 19b, which is toward the projection 17. When tightened, first a bolt 12 is passed through a hole 17 in the bottom 16 of the recess 15, and then pressed the washer part 12 a of the bolt 12 into the boss 19, until washer part 12 a is pressed into a space between the boss 19a and the bottom 16, and the bolt 12 is locked temporally into the recess 15.