A polyurethane-modified epoxy resin, which is obtained by modifying a bisphenol-based epoxy resin (a) represented by the following formula having an epoxy equivalent of from 150 g/eq to 200 g/eq and a hydroxyl equivalent of from 2,000 g/eq to 2,600 g/eq with a medium to high molecular weight polyol compound (b) having a number-average molecular weight of 200 or more, a polyisocyanate compound (c), and a low molecular weight polyol compound (d) which serves as a chain extender, and is a polyol represented by formula HO-R 5 -OH in the formula 13, Rs represents an alkylene group represented by the formula 13a, and g represents a number of from 1 to 10; the polyurethane-modified epoxy resin comprising a polyurethane having the epoxy resin (a) added to each of both terminals thereof and/or one terminal thereof, the polyurethane being obtained by: producing a urethane prepolymer by using the epoxy resin (a) in an amount of from 20 wt% to 60 wt% with respect to a total amount of the components (a), (b), (c), and (d), and causing the medium to high molecular weight polyol compound (b) and the polyisocyanate compound (c) to react with each other in a presence of the epoxy resin (a) while using the compounds in such amounts that a molar ratio between primary OH groups of the component (b) and NCO groups of the component (c) falls within a range of from 1:1.5 to 1:3; and then subjecting the urethane prepolymer to a polyurethanation reaction by adding the low molecular weight polyol compound (d) so that a molar ratio between NCO groups of the urethane prepolymer and OH groups of the low molecular weight polyol compound (d) may fall within a range of from 0.9:1 to 1:0.9: where R 1 represents H or a methyl group, and a represents a number of from 0 to 10, wherein the epoxy resin (a) is a bisphenol A-type epoxy resin represented by formula where a1 represents a number of from 0 to 10; wherein the epoxy equivalent is measured in accordance with JIS K 7236, and the hydroxyl equivalent is measured as disclosed in the description. producing a urethane prepolymer by using the epoxy resin (a) in an amount of from 20 wt% to 60 wt% with respect to a total amount of the components (a), (b), (c), and (d), and causing the medium to high molecular weight polyol compound (b) and the polyisocyanate compound (c) to react with each other in a presence of the epoxy resin (a) while using the compounds in such amounts that a molar ratio between primary OH groups of the component (b) and NCO groups of the component (c) falls within a range of from 1:1.5 to 1:3; and then subjecting the urethane prepolymer to a polyurethanation reaction by adding the low molecular weight polyol compound (d) so that a molar ratio between NCO groups of the urethane prepolymer and OH groups of the low molecular weight polyol compound (d) may fall within a range of from 0.9:1 to 1:0.9: where R 1 represents H or a methyl group, and a represents a number of from 0 to 10, wherein the epoxy resin (a) is a bisphenol A-type epoxy resin represented by formula where a1 represents a number of from 0 to 10; wherein the epoxy equivalent is measured in accordance with JIS K 7236, and the hydroxyl equivalent is measured as disclosed in the description. where R 1 represents H or a methyl group, and a represents a number of from 0 to 10, wherein the epoxy resin (a) is a bisphenol A-type epoxy resin represented by formula where a1 represents a number of from 0 to 10; wherein the epoxy equivalent is measured in accordance with JIS K 7236, and the hydroxyl equivalent is measured as disclosed in the description.