A portion of an outer periphery of a plug body of a forceps plug is cut out to form an arm having a shape along the circumferential direction of an outer peripheral surface of an opening. A tip portion of the arm is formed with a boss that protrudes toward the opening. An outer peripheral surface of the opening tip portion is formed with a cam groove for a forceps plug that guides the boss. The cam groove for a forceps plug is constituted by a cam groove for attachment that guides the boss 33 when the forceps plug is attached, a cam groove for locking that locks the boss, and a cam groove for removal that guides the boss when the forceps plug is removed. When the boss moves along the cam groove for removal, the deflection of the arm exceeds an elastic limit, and the arm breaks.