PURPOSE: To prevent the surface accuracy of a cut part from being deteriorated that is caused by flash or a burr occurring at the time of gate cutting.

CONSTITUTION: The assembly comprises a fixed mold 11 installed to a fixed platen, a movable mold 12 installed to a movable platen for forming a cavity with the fixed mold 11, and a stamper 38 mounted on the mold 11 on one side. A gate cutter 21 is retractably provided in the mold 12 on the other side. The gate cutter 21 forms a through hole of a molded piece when moving forward through the cavity. At this time, a flash or burr occurs by gate cutting, but it occurs on a molded piece on the side of the mold 11 with the stamper 38 mounted. Therefore, a cut part of the molded piece formed by gate cutting can be improved in the surface accuracy.