The gun-main-body side is provided with a gun main body-side connection part (24). The barrel side is provided with a barrel-side connection part (25). The gun main body-side connection part and the barrel-side connection part are connected to each other by an insertion structure. The gun main body-side connection part is provided with an inner circumferential surface (26) formed extending inwardfrom a connection opening. The barrel-side connection part is provided with an outer circumferential surface (27) formed so as to fit to the inner circumferential surface. At least one from among theinner circumferential surface and the outer circumferential surface is formed into a tapered shape. The other from among the inner circumferential surface and the outer circumferential surface is formed into a shape capable of moving along the tapered shape. The gun main body-side connection part and the barrel-side connection part are pulled in a direction along a center line by a coupling means(35), the inner circumferential surface or the outer circumferential surface which has the tapered shape, and the shape capable of moving therealong are fitted together, and the barrel is centered.