An epoxy resin composition comprising (A) an epoxy resin, (B) a curing accelerator, and (C) an inorganic filler is light transmissive when it satisfies formulae (1) and (2): [{2(n.sub.A.sup.2 +n.sub.C.sup.2)-(n.sub.A +n.sub.C).sup.2 }/2].sup.1/2 <3.0.times.10.sup.-3 (1) [{2(f.sub.A.sup.2 +f.sub.C.sup.2)-(f.sub.A +f.sub.C).sup.2 }/2].sup.1/2 <1.0.times.10.sup.-5 (2) wherein n.sub.A is the refractive index at T.sub.1.degree. C. of the cured unfilled composition, n.sub.C is the refractive index at T.sub.1.degree. C. of the inorganic filler, f.sub.A is a temperature coefficient of the refractive index of the cured unfilled composition, and f.sub.C is a temperature coefficient of the refractive index of the inorganic filler. The cured composition has improved heat resistance, humidity resistance and low stress as well as high transparency over a wide temperature range. The composition is suited for the sealing of optical semiconductor devices.