PURPOSE: An epoxy resin composition comprising a specified component and a high-reliability resin-sealed semiconductor device excellent in moisture resistance, high-temperature electrical properties and thermal-change resistance prepared therefrom.

CONSTITUTION: An epoxy resin composition is prepared by using (A) an epoxy resin (e.g., novolak epoxy resin of an epoxy equivalent of 170W300), (B) a molten mixture of a phenolic hydroxyl group-containing polymeric substance and an epoxy compound (the mixing ratio between both is preferably such that 0.001W 50 epoxy groups are present per 100 phenolic hydroxyl groups), and (C) an organic phosphine compound (e.g., triphenylphosphine, the amount is preferably such that 0.001W20wt% component C is present per total of components A and B) as essential components. Then, semiconductors are sealed with the above composition to obtain resin-sealed semiconductor devices of high quality and high reliability.