To provide a closing plug which effectively attains the easiness of installation and enhanced holding function.

The closing plug 1 includes a shaft body 21 to be inserted into an opening to close the opening, the shaft body having a diameter smaller than the opening diameter ka of the opening; a first claw portion 22 protruding outward from the shaft body 21 up to a diameter 22a smaller than the opening diameter ka; and a second claw portion 23 provided on the base end side beyond the first claw portion 22 with a space of a predetermined dimension therefrom, and protruded radially outward from the shaft body 21 up to a diameter larger than the opening diameter ka, the second claw portion being received within the opening when distorted to the base end side, and overlapped on the first claw portion 22 from the outside, when distorted to the tip side, with the diameter x in the overlapped position being larger than the opening diameter ka.