A heat-curable resin composition that yields a cured product having excellent heat resistance and light resistance, and a premolded package molded using the composition. The heat-curable resin composition comprises components (A) to (E) listed below: (A) an isocyanuric acid derivative containing at least one epoxy group within each molecule, in an amount of 100 parts by mass, (B) a silicone resin containing at least one epoxy group within each molecule, in an amount of 10 to 1,000 parts by mass, (C) an acid anhydride curing agent, in an amount such that the ratio of [total epoxy group equivalents within component (A) and component (B)/carboxyl group equivalents within component (C)] is within a range from 0.6 to 2.2, (D) a curing accelerator, in an amount within a range from 0.05 to 5 parts by mass per 100 parts by mass of the combined mass of components (A), (B) and (C), and (E) an inorganic filler, in an amount of 200 to 1,000 parts by mass per 100 parts by mass of the combined mass of components (A), (B) and (C).