To obtain the subject composition extremely low in generation of hydrogen and carbon monoxide when cured, excellent in productivity and reliability, and useful for sealing semiconductor devices by including an epoxy resin, a specific phenolic resin curing agent, a specific oxidizing agent, or the like.

This composition is obtained by essentially compounding (A) an epoxy resin, pref. a dicyclopentadiene-type epoxy resin of the formula ((n) is 0-10), (B) a phenolic resin curing agent having two or more hydroxyl groups in one molecule (e.g. a phenol-aralkyl resin), (C) a curing promoter, pref. e.g. an adduct of triphenylphosphine to benzoquinone, (D) an oxidizing agent consisting of at least one oxide selected from CuO, Ag2O, PbO2, Bi2O3 and MnO2, and (E) an inorganic filler such as fused silica, pref. in the equivalent ratio A/B of (0.7-1.3), wherein the component D accounts for 0.5-30 wt.% of the whole composition.