PROBLEM TO BE SOLVED: To prevent reuse of a plug body by allowing the plug body to be easily detached from a mouth ring.SOLUTION: On a lower end part inner peripheral surface of a frame body 45, two fixing pawls are provided at a pitch of 180° in a circumferential direction. By using the fixing pawl, a plug body 21 is attached to a flange of a moth ring 20 of a channel opening 17. On a lower end part of the frame body 45, a grip part 52 for separation is provided in a radially projected manner. At a connection part of the grip part 52 for separation to the frame body 45, a notch 70 is formed in a direction of a cylindrical center CL1. A connection part whose length in the direction of the cylindrical center CL1 is shortened by the notch 70 is formed as a fragile part 71. At an upper part of the grip part 52 for separation, a connection opening 55a and a side face opening 55 are formed. When the grip part 52 for separation is lifted and broken from the fragile part 71, the openings 55a and 55 are opened and largely changed in shape, making re-mounting impossible.
