Abstract: A process for manufacturing a door trim assembly in which resin-made door trim parts are automatically fixed to a resin-made door trim main body; and a manufacturing line therefore. Resin-made door trim parts and transformable members associated therewith are tentatively fitted to a resin-made door trim main body. In transforming processing unit (3) of manufacturing line, electrically operated cylinder (10) equipped with ultrasonic oscillator (2) extends and contracts in the direction of upper limit and is ratable within horizontal plane by means of ball screw mechanism (12). To the transformable members tentatively fitting resin-made door trim parts to resin-made door trim main body, the ultrasonic oscillator (2) comes close in opposite relationship by driving of the electrically operated cylinder (10) and ball screw mechanism (12), and performs ultrasonic irradiation.