A high-tolerance molding device that uses a pulling technique to quickly form a desired elongated hole in a workpiece such as a pipe or a sheet with little material loss, due to the use of single-piece molding, while preventing material degradation. The workpiece is provided with a pilot hole that is smaller than but roughly concentric with the desired elongated hole. This molding device includes a power-coupling arm that can be inserted into the pilot hole; a control unit that controls the movement of the power-coupling arm; and a molding plug that is prepositioned on the opposite side, with respect to the power-coupling arm, of the surface of the workpiece in which the pilot hole is provided.