To make a constitution capable of continuously producing a packaging bag with excellent self-supporting properties having reinforcing means provided on sides of a flexible bag.

A fold 4a is formed in the longitudinal direction. A bottom film 4 with the side including the fold 4a notched is sandwiched between barrel films 2 and 3 at portions corresponding to both sides of the packaging bag. The barrel films 2 and 3, and the bottom film 4 are overlaid. Side seals 16 are formed by heat-sealing between the overlapping barrel films 2 and 3 and between the barrel films 2 and 3 and the bottom film 4 in the direction orthogonal to the longitudinal direction of the film at both sides of the respective notches 5 of the bottom film 4. A reinforcing member 6 is inserted into a non-sealed portion 17 formed between the side seals 16 and 16 on both sides of the notch 5. The reinforcing member 6 is heat-welded to the barrel films 2 and 3.