Epoxy resin modified with polyurethane in low concentration, production method therefor, epoxy resin composition, and cured object

A low-concentration polyurethane-modified epoxy resin comprising a polyurethane having a bisphenol-based epoxy resin (a) represented by the following Formula (1) added to both terminals and/or a single terminal thereof,

• wherein the epoxy resin (a) having an epoxy equivalent of 150 to 200 g/eq and a hydroxyl equivalent of 2,000 to 2,600 g/eq is modified with a middle- and high-molecular-weight polyol compound (b) having a number average molecular weight of 200 or larger, a polyisocyanate compound (c), and a low-molecular-weight polyol compound (d) as a chain extender having a number average molecular weight of less than 200, and
• wherein it is obtained such that the epoxy resin (a) is used in an amount of 68 to 92 weight % based on a total amount of the components (a), (b), (c), and (d), the middle- and high-molecular-weight polyol compound (b) and the polyisocyanate compound (c) are used in such amounts that a molar ratio of OH groups in the component (b) to NCO groups in the component (c) is within a range of 1:2 to 1:30 and are reacted with each other in the presence of the epoxy resin (a) to produce a urethane prepolymer (P), and then, the low-molecular-weight polyol compound (d) is added to the urethane prepolymer (P) so that a molar ratio of NCO groups in the urethane prepolymer (P) to OH groups in the low-molecular-weight polyol compound (d) is within a range of 0.9:1 to 1:0.9 to cause a polyurethane reaction,
• where, R and R1 each independently represent H or a methyl group, and a is a number of 0 to 10.