Because of a clearance between a barrel supporting member and a barrel, a rotational movement of the barrel is not always constant. Thus, trajectory of a bullet is misaligned. A counterweight 40 is located upper than a barrel 10, and rings 61, 62, which are support portions, are located nearer to the counterweight 40 than a gravity center of the barrel 10. Since the barrel 10 is located lower than the counterweight 40 and engaged with a lower surface side of a gear 30, when a bullet is shot and the barrel 10 is moved backward, a force to rotate the barrel 10 itself clockwise is imparted by a circular surface shape of the gear 30. The barrel 10 receives a force to be rotated clockwise from the gravity center position, the ring 61 and the ring 62, and the barrel 10 is in contact with the inner surface of the barrel supporting member 50. Thus, in spite of the existence of the clearance, the barrel 10 is not displaced in the barrel supporting member 50.