A male belt-shaped base having a male member with a short width, and a separation strip are heat-sealed to a film. A female belt-shaped base having a female member with a long width is heat-sealed at a predetermined position so that the male portion and the female portion are at engageable positions. The film is layered and suitably sealed to form a zipper-tape bag. The film and the separation strip are cut to form a cut portion at a non-bonded region in which the film of the zipper-tape bag is not bonded, thereby forming an unsealing tab. The unsealing tab can be easily formed by simply cutting the film.