PURPOSE: To uniformize the thickness of a flat hollow container main body in the peripheral direction thereof by a method wherein a resin is allowed to flow out of a cylindrical resin injection passage to form the neck part of a container by injection molding and a tube is extruded while the neck part is separated from the injection passage to be subjected to blow molding and, at this time, the width of the resin injection passage in the diameter direction thereof is differentiated in the peripheral direction thereof.

CONSTITUTION: An accumulator chamber 6 storing a molten resin is formed in a die 4 having a nozzle 3 provided thereto and a closing piston 14 is arranged in the accumulator chamber 6 to form a resin injection passage 10. The nozzle 2 is formed so as to have a circular cross section and the closing piston 14 is formed so that the foremost end part 14a thereof is formed into a circular cross section becoming small in its diameter toward an outlet but the part 14b adjacent to the accumulator chamber 6 of said piston is formed so as to have an oval cross section and, at the time of the molding of a parison, the thickness of the region of the parison becoming the bent part of a container main body is made larger than other part. An injection mold 22 molding the neck part of a container is connected to the nozzle of the die and the resin in the accumulator chamber is injected by the extrusion piston 18 provided on the rear end of the die and extruded by the extrusion piston 18 at the same time when a mold is separated from the die and the parison 46 continuing to the neck part is formed to be subjected to blow molding.