An apparatus for successively making plastic bags each of which includes panel portions, side gusset portions and a zipper portion, the apparatus comprising: a panel material feeding device (10, 11, 12) by which a web of panel material is fed longitudinally thereof: and a panel material guide device (21, 22, 30, 31, 32, 33) by which the web of panel material is guided when being fed to be folded and folded back along longitudinal fold lines (14, 15, 16) and longitudinal fold back lines (17, 18) into superposed two layers having opposite side edges at each of which a folded portion (19, 20) is formed, the folded portion (19, 20) being folded into halves and interposed between the layers, the apparatus being characterized by a slitting device by which the web of panel material is slit along the longitudinal fold line into two divided parts (9a, 9b) after being folded and folded back so that one of the divided parts should include the folded portions (19, 20) formed therein; a divided parts guide device (37, 38, 39) by which the divided parts (9a, 9b) are guided when being fed to be separated and spaced from each other, fed as they are and then superposed with each other; a splitting and removing device (41, 42) by which the folded portions (19, 20) are split at positions into split portions and partially removed at the positions after the divided parts (9a, 9b) are separated and spaced from each other so that the split portions should be spaced from each other; a zipper material supplying device by which a body of zipper material is supplied onto 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 (9a, 9b) when the divided parts (9a, 9b) are superposed with each other; a longitudinal seal device by which the divided parts (9a, 9b) are heat sealed with each other longitudinally thereof after being superposed with each other; and a cross seal device by which the divided parts (9a, 9b) and the body of zipper material are heat sealed with each other widthwise of the divided parts (9a, 9b) after the divided parts (9a, 9b) are superposed with each other, the panel portions, the side gusset portions and the zipper portion being formed by the divided parts (9a, 9b), the folded portions (19, 20) and the body of zipper material. a panel material feeding device (10, 11, 12) by which a web of panel material is fed longitudinally thereof: and a panel material guide device (21, 22, 30, 31, 32, 33) by which the web of panel material is guided when being fed to be folded and folded back along longitudinal fold lines (14, 15, 16) and longitudinal fold back lines (17, 18) into superposed two layers having opposite side edges at each of which a folded portion (19, 20) is formed, the folded portion (19, 20) being folded into halves and interposed between the layers, the apparatus being characterized by a slitting device by which the web of panel material is slit along the longitudinal fold line into two divided parts (9a, 9b) after being folded and folded back so that one of the divided parts should include the folded portions (19, 20) formed therein; a divided parts guide device (37, 38, 39) by which the divided parts (9a, 9b) are guided when being fed to be separated and spaced from each other, fed as they are and then superposed with each other; a splitting and removing device (41, 42) by which the folded portions (19, 20) are split at positions into split portions and partially removed at the positions after the divided parts (9a, 9b) are separated and spaced from each other so that the split portions should be spaced from each other; a zipper material supplying device by which a body of zipper material is supplied onto 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 (9a, 9b) when the divided parts (9a, 9b) are superposed with each other; a longitudinal seal device by which the divided parts (9a, 9b) are heat sealed with each other longitudinally thereof after being superposed with each other; and a cross seal device by which the divided parts (9a, 9b) and the body of zipper material are heat sealed with each other widthwise of the divided parts (9a, 9b) after the divided parts (9a, 9b) are superposed with each other, the panel portions, the side gusset portions and the zipper portion being formed by the divided parts (9a, 9b), the folded portions (19, 20) and the body of zipper material.