A method of successively making plastic bags each of which includes panel portions, side gusset portions and a zipper portion, the method comprising:
- feeding a web of panel material longitudinally thereof;
- guiding the web of panel material when being fed to be folded and folded back along longitudinal folded lines and longitudinal folded back lines into superposed two layers having opposite side edges at each of which a folded portion is formed, the folded portion being folded into halves and interposed between the layers, the longitudinal folded lines comprising first, second and third folded lines, the first folded line extending near a first side edge of panel material, the longitudinal folded back lines comprising first and second folded back lines, the first folded back line extending between the first side edge of panel material and the first folded line, the second folded back line extending between the second and third folded lines, the web of panel material being folded along the first folded line and folded back along the first folded back line so that one of the folded portions is formed near the first side edge of panel material, the web of panel material being further folded along the second folded line, folded back along the second folded back line and folded along the third folded line so that the other folded portion is formed;
- slitting the web of panel material along the first, second and third folded lines into the folded portions and two divided parts after being folded and folded back, to make the folded portions and the divided parts divided from each other;
- guiding the divided parts when being fed to be separated and spaced from each other, fed as they are and then superposed with each other;
- fixing the folded portions and one of the divided parts to each other temporarily along slit lines after the divided parts are separated, to have temporarily fixed portions and unfixed portions formed alternately along the slit lines, the folded portions being then fed by the temporarily fixed portions;
- splitting the folded portions at the unfixed portion into split portions, and removing the folded portions at the unfixed portion partially after the divided parts are separated and spaced from each other so that the split portions are spaced from each other;
- supplying a body of zipper material onto the one of the divided parts to extend widthwise thereof, the body of zipper material being disposed between the split portions, the body of zipper material being then interposed between the divided parts when the divided parts are superposed with each other;
- heat sealing the divided parts with each other longitudinally thereof after being superposed with each other;
- and
- heat sealing the divided parts and the body of zipper material with each other widthwise of the divided parts after the divided parts are superposed with each other, the panel portions, the side gusset portions and the zipper portion being formed by the divided parts, the folded portions and the body of zipper material.