Provided are a toughness modifier for a curable resin which has good dispersibility in a curable resin containing a thermoplastic resin as well as in a cured product formed from a curable resin composition, and also has an excellent toughness-improving effect; and a curable resin composition containing the toughness modifier. The toughness modifier for a curable resin (D) is obtained by emulsion-polymerizing 5 to 50% by mass of a vinyl monomer (B) in the presence of 50 to 95% by mass of a rubber polymer (A) latex (calculated as the rubber polymer component) using 0.5 to 15 parts by mass of a nonionic reactive surfactant (C) (relative to 100 parts by mass of a total of (A) and (B)).