In one embodiment of this invention, a blow nozzle, which introduces blow air into a molded article in which a flange portion is supported by a blow cavity mold, includes a nozzle main body and an outer nozzle. The outer nozzle includes a first surface brought into press contact with the flange portion and a second surface disposed further outwards in a radial direction than the first surface and being brought into contact with the blow cavity mold. From when the nozzle main body is moved and the first surface is brought into contact with the flange portion to when the second surface is brought into contact with the blow cavity mold and the movement of the nozzle main body is stopped, the first surface of the outer nozzle is overdriven so that the flange portion is airtightly sealed by the first surface.