An apparatus for successively making plastic bags each of which includes panel portions and a bottom gusset portion, the apparatus comprising: panel material feeding means (9, 10) by which upper and lower webs of panel material (1, 2) can be superposed on each other and fed longitudinally thereof, the webs of panel material (1, 2) having outer surfaces and opposite side edges; bottom gusset material supply means by which a web of bottom gusset material can be supplied to one of the webs of panel material to be superposed on the outer surface thereof, the web of bottom gusset material having opposite side edges one of which extends along ones of the opposite side edges of the webs of panel material (1, 2); temporarily fixing means by which the web of bottom gusset material and the other web of panel material can be temporarily fixed to each other at respective side edges thereof after the webs of panel material (1, 2) have been superposed and the web of bottom gusset material has been supplied; guide means by which the one of the webs of panel material to which the bottom gusset material was superposed or said other web of panel material and the web of bottom gusset material temporarily fixed thereto can be guided so that said one of the webs of panel material or said other web of panel material can be folded at a position adjacent to said ones of the opposite side edges of the webs of panel material (1, 2) when the webs of panel material (1, 2) are fed and after the web of bottom gusset material and said other web of panel material have been temporarity fixed, the guide means being arranged such that, after the folding, the web of bottom gusset material extends between said ones of the opposite side edges of the webs of panel material (1, 2), the web of bottom gusset material being combined with the webs of panel material (1, 2); and longitudinal seal means by which the webs of panel material (1, 2) and the web of bottom gusset material are heat sealed with each other along said ones of the opposite side edges of the webs of panel material (1, 2), the panel portions being formed by the webs of panel material (1, 2), the bottom gusset portion being formed by the web of bottom gusset material. panel material feeding means (9, 10) by which upper and lower webs of panel material (1, 2) can be superposed on each other and fed longitudinally thereof, the webs of panel material (1, 2) having outer surfaces and opposite side edges; bottom gusset material supply means by which a web of bottom gusset material can be supplied to one of the webs of panel material to be superposed on the outer surface thereof, the web of bottom gusset material having opposite side edges one of which extends along ones of the opposite side edges of the webs of panel material (1, 2); temporarily fixing means by which the web of bottom gusset material and the other web of panel material can be temporarily fixed to each other at respective side edges thereof after the webs of panel material (1, 2) have been superposed and the web of bottom gusset material has been supplied; guide means by which the one of the webs of panel material to which the bottom gusset material was superposed or said other web of panel material and the web of bottom gusset material temporarily fixed thereto can be guided so that said one of the webs of panel material or said other web of panel material can be folded at a position adjacent to said ones of the opposite side edges of the webs of panel material (1, 2) when the webs of panel material (1, 2) are fed and after the web of bottom gusset material and said other web of panel material have been temporarity fixed, the guide means being arranged such that, after the folding, the web of bottom gusset material extends between said ones of the opposite side edges of the webs of panel material (1, 2), the web of bottom gusset material being combined with the webs of panel material (1, 2); and longitudinal seal means by which the webs of panel material (1, 2) and the web of bottom gusset material are heat sealed with each other along said ones of the opposite side edges of the webs of panel material (1, 2), the panel portions being formed by the webs of panel material (1, 2), the bottom gusset portion being formed by the web of bottom gusset material.