To efficiently mold a disc board having a favorable molding appraisal by a method wherein a molding material, the amount of which exceeds the volume of a cavity, is filled by injection in the cavity under the pressure, which exceeds a molding clamping force, so as to detect the amount of a mold opening under increasing in order to perform a gate cutting on the basis of a detected amount of the mold opening.

A cavity 3 is formed by matching molds 1 and 2 with each other. The molds 1 and 2 are clamped with a predetermined mold clamping force. A molding material, the amount of which exceeds the volume of the cavity 3 is filled by injection in the cavity 3 under the pressure, which exceeds a mold clamping force, so as to detect the amount L of a mold opening with a detecting means 7. A controlling means 8, in which the amount of the mold opening set in advance such as 30-70μm is stored, compares the amount L of the mold opening detected with the detecting means 7 with the set amount of the mold opening so as to perform a gate cutting by advancing a male cutter 19 for fitting in a female cutter 18, when the detected amount L of the mold opening exceeds the set amount of the mold opening.