A mouthpiece includes: a mouthpiece main body portion including a tubular member to which an air supply device is connected on one end side and a tire valve is connected on the other end side; an annular protruding portion which is formed at a required position on the outer circumferential portion of the mouthpiece main body portion; a tire valve insertion hole provided in a tire-valve-side tip portion of the mouthpiece main body portion; an O-ring arranged on the air supply device side inside the tire valve insertion hole; and a back-up ring that presses the O-ring from the air supply device side of the O-ring; an actuation body having a substantially convex shape in which a cylindrical tip portion is reduced in diameter.