PURPOSE: To eliminate the needs for finishing treatment by a method in which a degassing hole formed in the parting line of molds for injection molding is led to a gas trap cavity through a communication path having a circularly and gradually widened width in the counterpart mold.

CONSTITUTION: A degassing hole 11 of a pinpoint form is formed in the cavity 6 in the parting line 3, and the hole 11 is led to a gas trap cavity 10 formed in the parting line 3 through a communication path 12 having a circularly bent and gradually widened width through a core 8 in the movable mold 5. During the injection molding period, gas generated in the cavity 6 is easily dischrged to the cavity 10, and resin leaked from the hole 11 into the communication path 12 is simply cut off by the hole 11 of a pinpoint form. Therefore, no gas trap trace and gas burning occurs, no whitened cut portion is left behind, and the need for finishing treatment is eliminated.