A discharging nozzle is provided in an inside space of a metal cylinder for supplying an assist gas from an inside of the metal cylinder. The gas discharging nozzle shuts a high density energy beam intruding into the inside space of the metal cylinder through a hole, so that the energy beam is prevented from being emitted to an opposite side inner surface of the metal cylinder. Dross formed during the cut-machining is drifted and swept to the outside by an assist gas flowing out from the inside space of the metal cylinder through the hole, and is blown out by the assist gas flowing on an outside surface of the metal cylinder around the hole, thereby preventing the dross from sticking to peripheral area of the hole.