PROBLEM TO BE SOLVED: To provide a resin container molding method that allows a resin container, molded by a biaxial stretch blow molding method, to have a high drainage property and high heat resistance while forming upper and lower mouth parts therein, and to provide a preform and a resin container.SOLUTION: In a resin container molding method where a preform 1 for forming a resin container 3, mounted into a mold 4, is vertically stretched by a stretch rod 9 and horizontally stretched by blow air 14, the preform 1 has a cylindrical shape extending vertically as a whole and has a body part 15 between mouth parts 10, 12 formed on both upper and lower sides thereof, the wall thickness of the body part 15 gradually decreases from a vertical central part toward upper and lower directions, the wall thickness of a front and rear part of the central part is larger than that of a left and right part thereof, the difference between the wall thickness of the front and rear part and that of the left and right part becomes smaller from the central part toward upper and lower directions, and the body part 15 is formed into a flat shape where the left and right width is larger than the front and rear width by performing biaxial stretching of the preform 1 so that the vertical stretching precedes the horizontal stretching.
