The plug includes: a resilient member that is provided on the valve body and serves to bias the valve body in a closing direction; a valve body cover attached to the valve body in order to prevent the resilient member from coming of a rotation member rotatably provided on the valve body cover; at least two engagement claw portions that are provided on the rotation member and serve to allow the valve body cover to be engaged with an upper portion of the valve body; and claw guiding grooves and the recessed grooves serving as steps that are provided on an upper portion of the valve body, the number of the claw guiding grooves being equal to the number of or the integral multiple of the engagement claw portions, the recessed grooves allowing the rotation member to rotate.