Abstract: A method for manufacturing a hollow air spoiler comprising the steps of setting upper and lower parts molded from thermoplastic synthetic resin material by the injection molding method to upper and lower fixed jigs, placing a heater device between the upper and lower parts, pressurizing the upper and lower fixed jigs by bringing them together under pressure, contacting the outer layers of end faces of the upper and lower parts to the heater device under pressure, opening the upper and lower fixed jigs at the time point it has reached a desired melting degree from the outer layers of the end faces toward inside, removing the heater device from the upper and lower fixed jigs, closing the upper and lower fixed jigs, maintaining the pressure until it has reached to a desired dimension directing inwardly from the outer layers of the end faces, opening the upper and lower fixed jigs to remove a product therefrom at the time point the molten thermoplastic synthetic resin material is fixed and united, removing an out