PURPOSE: To enable to give the orientation effect to a parison only by attaching a simple device by a method wherein air or refrigerant is blasted from the nozzles of a cooling ring against the parison just after extrusion and having a temperature higher than its melting point so as to lower the temperature of the parison down to the specified temperature.

CONSTITUTION: Polypropylene, for example, is kneaded and melted at an extruder 1 so as to continuously extrude a parison 3 through a die head 2. Cooling air or refrigerant is blasted through an infinite number of small holes provided on a cooling ring 4 side at all times so as to lower the temperature of the parison down to the melting point or lower or to the vicinity of the heat deformation temperature of glass transition point of the parison. At the same time, the parison 3, the temperature of which is lowered, is held between molds 5, a needle 7 is stuck in the parison. Compressed air is blown through the needle 7 into the parison so as to form the parison into the form of a mold cavity. When completely cooled, the molds 5 are opened so as to remove a molded part from the molds.