PURPOSE: To enable to fusion-weld a large number of plastic sheets, which are overlapped with each other, at one time, by a method wherein a plual number of molds is located between an upper and a lower electrode board of a high-frequency welder, a plural number of thermoplastic sheets is situated between the molders, and high-frequency power sources being different in a magnetic pole are connected to the adjoining molds.

CONSTITUTION: In case an accordeon type bag is manufactured by the use of a plastic sheet serving as a stock, a large number of "ko"(Japanese:katakana,2533)-shaped molds 4, 4'... are located between a lower electrode board 1 and an upper electrode board 6 of a high-frequency welder, gusset-forming segments 3, 3'... are wound around the molds, and partition sheets 5, 5'... are situated between the molds 4 and 4', and 4"..., respectively. When they are adhered together by pressure, high-frequency power sources, being different in magnetic pole, simultaneously, are conected to the adjoining molds 4, 4'... to apply a voltage, and this permits the formation of an accordeon type bag U. In case the plastic sheet is not fused between some molds, said adjoining molds are connected to the same magnetic pole or connected to no magnetic pole. Or an insulating body is located between the two molds.