orifice member is greater than or equal to 2 mm a pressure loss of the gas at the orifice member is significantly produced thus no sonic gas flow is formed unless the gas pressure on the upstream side of the orifice is set to an extremely high pressure higher pressure is required to form the supersonic flow a gas supply unit such as a regulator of a commonly used gas cylinder is set to a gas pressure corresponding to about 5 atmospheric pressures at most therefore the thickness of the orifice may desirably be 1 5 mm or less on the other hand a problem arises in that when the thickness of the orifice member is less than or equal to 0 1 mm it breaks from the viewpoint of strength it is therefore desirable that the thickness of the orifice member ranges from 0 1 mm to 1 5 mm in the present embodiment the thickness of the orifice member was set to 0 2 mm and the gas pressure at the gas supply unit was set to 5 atmospheric pressures the spray gas is injected through the clearance space between the orifice and the tube the volume corresponding to annular.