PURPOSE: To make the heat treatment easy, by a method wherein a heat-treating metal mold and a heating air blow core are arranged between the orientation blow molder station and the unloading station in the injection orientation blow molder intermittently rotating, and the heat treatment of blow moldings is carried out in succession to the orientation injection blow molding.

CONSTITUTION: A bottomed parison is molded in an injection molding station A and the bottomed parison is transferred to a temperature regulation station B by holding in a neck-mold. Thereafter, it is transferred to the orientation blow station C and orientatively blow-molded to the blow moldings 15. Then, the blow molding 15 is held in the neck-mold and transferred to the inside of heat-treating mold of the secondary processing station D. High temperature air is blown into the blow molding 15 with the heating air blow core 19 and pressurized, and the heat-treatment is carried out by keeping the condition of adhesion to the cavity for a fixed time. Thus, the heat treatment can be easily carried out.