PROBLEM TO BE SOLVED: To suppress molding defects such as discoloration/denaturation of a compact called surface carbonization and caused by air and organic polymer material decomposition gas (outgas) present in a die and insufficient charging in the die, in injection molding of an organic polymer material compact having a complicated shape.SOLUTION: A port 9 to introduce a gas for preventing molding defects is installed at an optional location in a die for molding an organic polymer material. After a step for introducing a nonflammable gas from the introduction port into a die at a pressure of 0.1 kg/cm, G or higher and 10 kg/cmor less, and less than G, the die is depressurized and then molten organic polymer material is charged in the die while keeping the pressure in the die at normal pressure. The generation of outgas is thus greatly reduced to effectively suppress surface carbonization of a compact and insufficient charging, in injection molding of polybutylene terephthalate practiced under the condition
