PURPOSE: To make it possible to mechanically remove flashes of complicated shape by allowing combustion gas to flow out of an outer nozzle of a double nozzle consisting of inner and outer nozzles at a temperature at which thermoplastic resin melts and allowing air or hot air to flow out of an inner nozzle toward the roots of the flashes.

CONSTITUTION: Combustion gas is caused to flow out of an outer nozzle 13, simultaneously therewith air or hot air is caused to flow out from an inner nozzle 12 toward the control region of the combustion gas to promote combustion of the gas outside a double nozzle and increase dynamic pressure of the combustion gas. If the combustion gas (1500-3000°C) is directed to flashes D to be cut and moved along them, roots of the flashes are melted by heat of the gas and the melted resin is forcibly divided to the side of the flashes and the side of a molded item by the dynamic pressure of the air flowing out of the inner nozzle, so that gaps are formed between the flashes and the molded item, resulting in separation of the flashes from the item. Parts of the molded item wherefrom the flashes have been removed become smooth due to the flow of the molten resin.