A blow-molding method for manufacturing a hollow molded article comprising: setting an interval of a die slit in a die head according to a target wall thickness of the hollow molded article to be molded; extruding a molten resin in an accumulator into a cylindrical shape from the die slit to form a foamed parison; and molding the foamed parison in a mold to obtain the hollow molded article, wherein - from start of the extrusion of the foamed parison through termination of the extrusion of the foamed parison, the interval of the die slit is gradually increased in consideration of the drawdown so that a thickness of the foamed parison gradually increases, and - as swelling foaming occurs when molding the thick hollow molded article, the extrusion is started with the interval of the die slit being set at a smaller value than a value set according to the target wall thickness, and then the interval of the die slit is gradually increased to match the value set according to the target wall thickness, characterized in that - immediately prior to termination of the extrusion, the interval of the die slit is made larger than the value set according to the target wall thickness in consideration of said drawdown and the swelling foaming to increase the thickness of the foamed parison, such that rupture in the uttermost part of the foamed parison is prevented. setting an interval of a die slit in a die head according to a target wall thickness of the hollow molded article to be molded; extruding a molten resin in an accumulator into a cylindrical shape from the die slit to form a foamed parison; and molding the foamed parison in a mold to obtain the hollow molded article, wherein - from start of the extrusion of the foamed parison through termination of the extrusion of the foamed parison, the interval of the die slit is gradually increased in consideration of the drawdown so that a thickness of the foamed parison gradually increases, and - as swelling foaming occurs when molding the thick hollow molded article, the extrusion is started with the interval of the die slit being set at a smaller value than a value set according to the target wall thickness, and then the interval of the die slit is gradually increased to match the value set according to the target wall thickness, - from start of the extrusion of the foamed parison through termination of the extrusion of the foamed parison, the interval of the die slit is gradually increased in consideration of the drawdown so that a thickness of the foamed parison gradually increases, and - as swelling foaming occurs when molding the thick hollow molded article, the extrusion is started with the interval of the die slit being set at a smaller value than a value set according to the target wall thickness, and then the interval of the die slit is gradually increased to match the value set according to the target wall thickness, characterized in that - immediately prior to termination of the extrusion, the interval of the die slit is made larger than the value set according to the target wall thickness in consideration of said drawdown and the swelling foaming to increase the thickness of the foamed parison, such that rupture in the uttermost part of the foamed parison is prevented. - immediately prior to termination of the extrusion, the interval of the die slit is made larger than the value set according to the target wall thickness in consideration of said drawdown and the swelling foaming to increase the thickness of the foamed parison, such that rupture in the uttermost part of the foamed parison is prevented.