Method of molding a large returnable container comprising: a heat treatment step that includes intermediate blow molding a preform that has a thick wall and that is formed of a polyester resin, or a primary blow-molded article obtained by subjecting the preform to primary blow molding, into an intermediate molded article from which a strain that has occurred during blow molding has been removed in a heat treatment mold; and a final blow molding step that includes subjecting the intermediate molded article that has shrunk to final blow molding with heating to obtain the large returnable container, wherein the heat treatment step includes disposing the preform or the primary blow-molded article in the heat treatment mold, and pressurizing the preform or the primary blow-molded article by introducing high-pressure air into the preform or the primary blow-molded article so that a shoulder and a body that have been formed by blow molding closely contacts the cavity surface, and are subjected to a heat treatment, by contact with the cavity surface,that the shoulder being heated at a low temperature as compared with the body characterized in that the heat treatment mold includes a pair of split molds, each of the pair of split molds including a parting surface that is continuous with the cavity surface, the parting surfaces of the pair of split molds coming in contact with each other, each of the pair of split molds including a slit that becomes exposed on the cavity surface of the heat treatment mold so as not to reach the parting surface, and that the shoulder being heated at a low temperature as compared with the body, and that the heat treatment step includes separating and insulating heating zones that respectively correspond to the shoulder and the body of the primary blow molded article by an air insulation layer formed by a slit. a heat treatment step that includes intermediate blow molding a preform that has a thick wall and that is formed of a polyester resin, or a primary blow-molded article obtained by subjecting the preform to primary blow molding, into an intermediate molded article from which a strain that has occurred during blow molding has been removed in a heat treatment mold; and a final blow molding step that includes subjecting the intermediate molded article that has shrunk to final blow molding with heating to obtain the large returnable container, wherein the heat treatment step includes disposing the preform or the primary blow-molded article in the heat treatment mold, and pressurizing the preform or the primary blow-molded article by introducing high-pressure air into the preform or the primary blow-molded article so that a shoulder and a body that have been formed by blow molding closely contacts the cavity surface, and are subjected to a heat treatment, by contact with the cavity surface,that the shoulder being heated at a low temperature as compared with the body characterized in that the heat treatment mold includes a pair of split molds, each of the pair of split molds including a parting surface that is continuous with the cavity surface, the parting surfaces of the pair of split molds coming in contact with each other, each of the pair of split molds including a slit that becomes exposed on the cavity surface of the heat treatment mold so as not to reach the parting surface, and that the shoulder being heated at a low temperature as compared with the body, and that the heat treatment step includes separating and insulating heating zones that respectively correspond to the shoulder and the body of the primary blow molded article by an air insulation layer formed by a slit.