A thermosetting epoxy resin composition is provided comprising (A) a reaction product obtained through reaction of a triazine derived epoxy resin with an acid anhydride, (C) a reflective agent, (D) an inorganic filler, and (E) a curing catalyst. In one embodiment, (B) an internal parting agent having a melting point of 50-90° C. is included. In another embodiment, (I) inorganic whisker fibers having an average fineness of 0.05-50 μm and an average length of 1.0-1,000 μm are included.