Provided are a white heat-curable epoxy resin composition having a high strength, high toughness and superior heat resistance; and a semiconductor device whose light receiving and other semiconductor elements are encapsulated by a cured product of such composition. The composition includes: (A) a prepolymer obtained by heating, melting and mixing components (A-1) triazine derivative epoxy resin, (A-2) acid anhydride and (A-3) acrylic block copolymer so as to react them at an epoxy group equivalent in the component (A-1)/acid anhydride group equivalent of the component (A-2) ratio of 0.6 to 2.0; (B) a white pigment including at least a titanium oxide; (C) an inorganic filler; (D) a curing accelerator; and (E) an antioxidant.