PROBLEM TO BE SOLVED: To provide an urethane-modified epoxy resin, a curable composition, a cured product thereof, a fiber-reinforced composite material and a fiber-reinforced resin molded article excellent in storage stability and fracture toughness of the cured product.SOLUTION: There is provided the urethane-modified epoxy resin obtained by reacting (A) an isocyanate group-containing urethane resin obtained by reacting a polyol compound (a1) and a polyisocyanate compound (a2) with (B) a hydroxyl group-containing epoxy compound represented by structural formula (1), where Ar is a phenylene group, a naphthylene group, or a structural site having an alkyl group or an alkoxy group on these aromatic core and R is an alkylene group or an alkenyl group having 1 to 6 carbon atoms.
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