PURPOSE: To obtain an epoxy resin cast molding useful for electrical parts, etc., having good stability in insulating gas atmosphere, good heat resistance, good cracking resistance, from a solid epoxy resin, a curing agent, and a mixed filler of Al2O3 and AlF3, having a specific volumetric content, a specific composition, and a specific grain size distribution.

CONSTITUTION: An epoxy resin composition, consisting of an epoxy resin base composed of a solid (at ordinary temperature) epoxy resin (A) of an epoxy equivalent of 300 or more, an acid anhydride-based curing agent (B) for epoxy resin, and a curing accerator (C) as needed, and a mixed filler (D) composed of alumina and aluminum fluoride of a volumetric content of 5W50%, having such a grain size distribution that particles 5μ or less size are 15W50%, 10μ or less size is 15W50%, 20μ or less size is 40W80%, 30μ or less size is 60W95%, and 40μ or less size is 75W100%, and a volumetric content of 35W50% on the basis of all the composition, is prepared. The epoxy resin composition is cast to obtain a cast molding, e.g., an electrical part consisting of an insulator 11 and Al electrodes 12 and 13.