The production method of the present invention method of producing an air duct for the air conditioning apparatus of the vehicle according to the, (S1) homopolypropylene resin and the front end of the extruder through a hopper of at least one resin of high-density polyethylene resin or a medium density polyethylene resin extruder a first step of melting were charged into a unit; (S2) is connected to the front end of the extruder, were introduced into the blowing agent through a side feeder provided in the rear end portion of the extruder is maintained at a temperature lower than the front end portion and mixed with the melt of the first stage, and then extruded in a molten sheet form a second step; (S3) a third step of forming a foam by foaming the foaming agent embedded in the extruded sheet; And (S4) by putting into the foam molding machine comprises a fourth step of forming the air duct. An air duct for the air conditioner of the automobile molding according to the production method of the present invention, as well as to light and heat-insulating property is excellent, and contributes to improved light weight of the car air-conditioning efficiency, is excellent in mechanical properties, it is good in durability.