Abstract: A mold for injection molding including a cylindrical outer mold and a columnar inner mold. The inner mold is provided, on its external circumferential surface, with a large number of tooth-shaped grooves extending in the axial direction thereof, and it is placed inside the outer mold to define an annular cavity between the external circumferential surface of the inner mold and the internal circumferential surface of the outer mold. The outer mold is provided with a plurality of runners extending axially near the internal circumferential surface. The runners are arranged at regular annular intervals along the circumference of the outer mold. Furthermore, slit-shaped gates are formed along the full length of the runners, the gates opening into the internal circumferential surface of the outer mold, in such a way that the inflow of molten molding material flowing from each runner into the cavity is gradually reduced along the runner from the upstream side towards the downstream end.