PURPOSE: To prevent sealing dregs from adhering to a nichrome wire, and to produce quickly a plastic film having an enough sealing strength, by cutting the passage of an electric current to the nichrome wire in accordance with its position when a heat holder is driven.

CONSTITUTION: While a long plastic film is being folded once, it is fed onto a Teflon cloth 8 placed on a receiving table 2 with a rubber cushion 2a therebetween. When a heater holder 4 is lowered and it reaches a point C near the lowest poing B, the passage of an electric current to the nichrome wire 5 is started, and after the film 7 is melted and welded at the lowest point B, the passage of the electric current is cut, and the heater holder 4 is raised. When the heater holder is raised, an electric current is passed to the nichrome wire at the point C to heat it thereby burning the sealing dregs 7" adhered to the nichrome wire to be removed. At a point D, the passage of the electric current is cut, and it is raised to the highest position where it is waited.