A packaging bag is provided with a bag body and thermoplastic resin-made reinforcing members provided at the outer edge of the bag body. A window is provided in each reinforcing member. In forming the packaging bag, the bag body is placed between a pair of molds respectively having recesses to clamp these two molds so that a corresponding pair of the recesses are opposed to each other. Thereafter, in order to form the reinforcing members at the outer edge of the bag body, melted or softened thermoplastic resin is injected to fill cavities formed by the recesses of the thus clamped molds. Each of the cavities of the molds is formed so as to coincide with an outer shape of the corresponding reinforcing member. A projection extends from the bottom face of each recess at a part of the cavity corresponding to a part of each reinforcing member on which the window is provided. The outer edge of the bag body placed between the two molds is placed inside the cavities in a state of being held between the two projections.