To make it easy to mount a cap plugging a bottomed cylindrical hole and also make it difficult to remove the cap from the bottomed cylindrical hole without making an engagement hole or the like in the bottomed cylindrical hole.

The cap 20 includes a ceiling plate 22 arranged to cross the bottomed cylindrical hole 13, a cylindrical part 24 aligned coaxially with the ceiling plate 22 and fixed to the ceiling part 22 at its base end part, and a notch part 24s formed in the cylindrical part 24 and open on a front end side of the cylindrical part 24, wherein, by the cylindrical part 24 and the ceiling plate 22 being pressed to be inserted in the bottomed cylindrical hole 13 in an axial direction, the cylindrical part 24 and the ceiling plate 22 are pressed by an inner surface of the bottomed cylindrical hole 13 from outside in a radial direction to be deformed in a shrinking manner in the radial direction, thereby plugging the bottomed cylindrical hole 13.