PURPOSE: To obtain the bag of a stable shape by dividing the front side part and the rear side part of a bag main body into two and forming them by a forming mold, and forming a box structure by joining the front side part and the rear side part of the main body.

CONSTITUTION: A design of the whole of a bag main body is determined, a metallic mold for injection molding corresponding to its design is manufactured by dividing it into a front side part 1 and a rear side part 2, and by using this metallic mold, for instance, the front side part 1 and the rear side part 2 are formed by an ABS resin and a polypropylene resin, respectively. In this case, in order to make a plastic hinge part 4 in the metallic mold of the rear side part 2, a projecting bead in which a notch part can be formed is attached to about the center part desirably. This bead position can be set freely between 80% and 20% from under height of the rear side part 2. Subsequently, the front side part 1 and the rear side part 2 which are formed are combined by superposing the respective peripheral edge end parts 5, 6 and a box structure is formed. In such a way, the forming bag for children being stable structurally can be manufactured easily.