PURPOSE: To cut off a resin material reliably from the surface of a molded product by a method wherein a cutting-off piece provided with a runner and tab gate is arranged on a parting line surface of a movable mold and the cutting-off piece is made movable along the parting line surface through a pulling-out device of a hydraulic cylinder.

CONSTITUTION: A resin material 9 is cast through a hot sprue 3, hot runner 4 and hot tip 5 in a first step gate. The resin material 9 is pressed into a cavity 8 from a tab gate 7 through a secondary runner 6 as a second step gate from the hot tip 5. When the resin material 9 within the cavity 8 is cured, a cutting- off piece 12 at the second gate is pulled out in an outside direction of a mold along a parting line surface 10 by a drawing-out device 14 before mold break of a stationary mold 1 and movable mold 2. When the cutting-out piece 12 is pulled out, its tab gate 7 is separated from the lower fringe part of the cavity 8, a residual resin material within the tab gate 7 and the lower fringe part of a molded product 13 molded within the cavity 8 are sheared and cut off precisely along the parting line surface 10.