[Task] To perform correct centering of a barrel without generating inclination or rattling even in a state where a special precision device or method for assembling or the like is not prepared. [Solution] A gun body includes a gun body side connection portion 24, a barrel includes a barrel side connection portion 25, and the gun body side connection portion and the barrel side connection portion are connected to each other by an insertion structure. The gun body side connection portion includes an inner peripheral surface 26 which is formed inward from a connection opening, the barrel side connection portion includes an outer peripheral surface 27 which is formed to be fitted to the inner peripheral surface, at least one of the inner peripheral surface and the outer peripheral surface is formed in a tapered shape, and the other of the inner peripheral surface and the outer peripheral surface is formed in a shape which is movable along the tapered shape. The gun body side connection portion and the barrel side connection portion are drawn in a direction along a center line by coupling means 35, the inner peripheral surface or the outer peripheral surface having the tapered shape and the movable shape are fitted to each other, and the barrel is centered.