PURPOSE: To provide the titled resin composition containing an epoxy resin, a specific hardener, and hydrated amorphous silicon dioxide, having high performance, and giving a resin-encapsulated semiconductor device having high reliability.

CONSTITUTION: The objective composition contains (A) an epoxy resin (preferably novolac epoxy resin having an epoxy equivalent of preferably 170W300), (B) a curing agent having ≥2 phenolic hydroxyl groups in one molecule (preferably novolac phenolic resin) and/or an acid anhydride curing agent (e.g. maleic anhydride, succinic anhydride, etc.) and (C) preferably 0.1W10wt% hydrated amorphous silicon dioxide. The composition is preferably further incorporated with fused silica and/or crystalline silica as an inorganic filler.