To obtain a plastic bag which is excellent in appearance and can stand straight wherein a heat-sealed part need not be cut by melting to cut both layers of respective side gussets and both layers of a main body slantly with respect to both side rims of the main body, melting both layers of the main body between the respective side gussets at a right angle with respect to both side rims of the main body and joining them respectively.

The bag making machine has first hot blades 18, 19 for melting to cut a continuous gusset 13 and a continuous body 12, wherein their blade tips extend slantly toward both side rims 3 of the continuous body 12. One of layers of each continuous gusset 13 and one of layers of the continuous body 12 are overlaid and interposed between the upper hot blade 18 and a receive plate, while at the same time the other layer of the continuous gusset 13 and the other layer of the continuous body 12 are overlaid and interposed between the lower hot blade 19 and the receive plate, for melting to cut slantly 9, 10. Then a cutter 24 is used to cut the continuous body 12 and the continuous gusset 13 at a right angle with respect to both side rims 3 of the continuous body 12, and subsequently both layers of the continuous body 12 are melted to cut 11 by the second hot blade.