Abstract: In an epoxy resin composition comprising an epoxy resin, a curing agent, and at least 70% by weight of an inorganic filler, at least one of the epoxy resin and the curing agent has such a molecular weight distribution as to provide an average dispersity Mw/Mn of less than 1.6, a two-nucleus compound content of less than 8% by weight and a seven- and more-nucleus compound content of less than 32% by weight. When the composition is cured at 180.degree. C. for 90 seconds into a primary product having Tg1 and the primary product postcured at 180.degree. C. for 5 hours into a secondary product having Tg2, the relationship: (Tg2-Tg1)/Tg2<0.1 is satisfied. The composition is fast-curing and effectively moldable and cures into a reliable product without a need for postcure.