A heat-curable epoxy resin composition for optical semiconductor element encapsulation capable of being pressure molded and transfer molded under room temperature, comprising: (A) a prepolymer obtained by reacting at least one component of (A-1), (A-2) and (A-3); and (A-4), (A-1) a triazine derivative epoxy resin having not less than three epoxy groups in one molecule, (A-2) at least one epoxy resin that is non-fluid at 25° C. and selected from the group consisting of a bisphenol-type epoxy resin, a hydrogenated bisphenol-type epoxy resin, an alicyclic epoxy resin and monoalkyl diglycidyl isocyanurate, (A-3) a cyclic siloxane compound having not less than two epoxy groups in one molecule, and (A-4) an acid anhydride curing agent in a liquid state at 25° C.; (A′) a component(s) among (A-1) to (A-3) that is/are not used in said prepolymer (A); and (B) a curing accelerator.