To cool a blow molded article in a mold forcibly, uniformly and in a short time.

In a state wherein air is blown into a parison, mist is sprayed from the fore end of a mist nozzle 16 positioned inside a small-diameter pipe 20 and is brought into direct contact with the inside of a blow molded article, and after the inside of the blow molded article is cooled down quickly by the latent heat effect of evaporation, the spray is stopped. In a state wherein the fore end part of the mist nozzle 16 is merged and withdrawn into a base part 21 being a large-diameter part of an air blowing nozzle 10, subsequently, air is supplied and discharged by using an air supply port 11 and an air discharge port 12 provided in the peripheral side of the large-diameter base part 21 of the air blowing nozzle 10, while a blowing pressure of the air is maintained. Thereby the air inside the blow molded article is circulated and the inside of this blow molded article is cooled down by the air.