To provide a method for molding a composite insulator where heating temperature for vulcanization curing can be set high, molding time can be shortened, and which has no displacement of an casing to a core member, and which has good adhesiveness of the core member with the casing particularly in a tapered polymer insulator tube.

A core member 2 is put in a metal mold 1 for molding a composite insulator, and an casing forming material is filled into casing forming space 1a, 1b around the core member 2, further the metal mold 1 is heated at prescribed temperature, then the casing forming material is cured. Here, in the curing process of the casing forming material, the metal mold 1 is opened by minute distance, and internal pressure of the metal mold 1 is lowered.