To provide a curable epoxy resin composition from which a cured product can be formed, the cured product having high heat resistance, light resistance and thermal shock resistance, in particular, having excellent moisture-resistant reflow properties.

A curable epoxy resin composition includes: an alicyclic epoxy compound (A); a monoallyl diglycidyl isocyanurate compound (B) expressed by formula (1); a quaternary phosphonium salt (C); a curing agent (D); a curing accelerator (E); and a siloxane derivative that has two or more epoxy groups in a molecule, wherein a weight ratio of the quaternary phosphonium salt (C) and the curing accelerator (D) [quaternary phosphonium salt (C)/curing agent (D)] is 0.01 to 100. [In the formula, R₁ and R₂ each represents a hydrogen atom or an alkyl radical having a carbon number of 1 to 8].