The invention provides a manufacturing method of a packing bag with a self-closing drain. In the method, at least one lateral part of a tablet main component (240) at a folded line (244b) which extends along the transportation direction of the main component is folded up to form a branch part (254) in the packing bag through the tablet type. The tablet main component (240) is guided by a guiding element. Thus, the through line length of the part occupying any position along the transverse direction of the passing main component is basically kept constant from the beginning of the folding to the end of the folding during the folding process of the main component.