PURPOSE: To facilitate insertion so as to make a reliable seal function by forming a plug by an elastic material, and providing a projecting surface-like thin part on the center part, a ring groove on the thick bank part outer periphery surrounding it, and a sectional wedged part on the through hole side of the bank part.

CONSTITUTION: When a plug 2 is pressed from a bank part 20 outer periphery of the plug 2 to the plug 2 center direction by human power F1, a thick part 2a is bent to the spherical projecting surface side, and the diameter of a wedge part 20b is reduced. The bank part 20 is returned into the diameter increasing direction by releasing the applied force after the plug is inserted into a through hole 1a, and a plate member 1 inner peripheral end surface is fitted to a ring groove 2c, so as to cover the through hole 1a. A force in the outer peripheral direction by stiffness of rib parts 2d, 2e is added to tensile force and becomes a force F3 for pressing the bank part 20 in the outer peripheral direction, and the ring groove 2c and the through hole 1a inner peripheral end surface are firmly fitted to each other. Accordingly, the groove and the through hole can be easily and reliably bonded.