A molding system for assembling a part by moving a workpiece gripped by a hand of a robot toward a mold of an injection molding machine, while rotating a molded article supported on the mold and the workpiece relative to each other, whereby the molded article and the workpiece are screw-engaged. The molding system includes a rotation detector for detecting a rotational amount between the molded article and the workpiece, an assembled state judging unit for judging if the assembled part is in a predetermined state, and a control unit for relatively rotating and displacing the molded article and the workpiece until the rotational amount exceeds a predetermined rotational amount and the assembled state becomes the predetermined state.