To provide a flame-retarded resin composition that has a high degree of flame retardancy and excellent reliability with no addition of halogen compound.

This resin composition comprises (A) an epoxy resin that has at least two epoxy groups in one molecular, (B) a curing agent, and (C) an organophosphorus compound that has a chemical structure that at least one phosphorus atom covalently bonds to at least one carbon atom in one molecule and bears at least two hydroxy groups in one molecule. In this resin composition, the content of phosphorus in the resin composition is 0.3-5 wt.%.