To prevent the occurrence of projecting parts on the surface due to fine foreign matter by forming a pair of metallic dies used for the injection molding of a resin material from a material having a specific heat conductivity to suppress the formation of a skin layer on a part in contact with the dies.

The molten resin material is filled into a cavity 14 formed from the fixed side die 11, the movable side die 12 and an outer peripheral ring 13 and cured. The material such as zirconium oxide having <0.03 cal/ cm.sec.°C heat conductivity is used for a pair of the dies. As a result, the molten resin material is prevented from making the curing rate slow and from being rapidly cooled on a part in contact with the dies to eliminate the formation of the skin layer and because the resin material is gradually cured from a part in contact with the dies, the unhardened part is uniformly existed without hardening only a part, where the foreign matter is existed, even if the fine foreign matter is incorporated, the difference of shrinkage is mitigated and the generation of the projecting part on the surface is prevented.