The invention provides a method of inflation extrusion molding, comprising: introducing one or more resin materials extruded from one or more extruders into a die; subsequently extruding the resin material(s) from the die and inflating the extrudate into a cylindrical shape having a predetermined diameter by blown-film extrusion; and then passing the cylindrical inflated resin material through a space between stabilizers which face each other at a predetermined angle, to deform the cylindrical resin material into a flat tube shape having an elongated elliptic section, thereby producing a film or sheet having a predetermined width, wherein the deformation of the cylindrical inflated resin material into a flat tube shape having an elongated elliptic section is assisted by a guide device to thereby facilitate the deformation.