An epoxy resin composition contains (A) an epoxy resin, (B) a curing agent, (C) an inorganic filler, and (D) catalyzed microcapsules containing an imidazole compound or organic phosphorus compound and having a mean particle size of 0.5-50 &mgr;m, the quantity of the catalyst leached out from the microcapsules in o-cresol at 30° C. for 15 minutes being at least 70% by weight of the entire catalyst quantity. The composition is suited for semiconductor package encapsulation since it has satisfactory catalyst latency, storage stability and cure.