Provided are a polyurethane-modified epoxy resin composition having satisfactory operability of processing, such as casting or impregnation, in a composition state, a production method therefor, and a composition thereof. The polyurethane-modified epoxy resin is obtained by modifying a secondary hydroxyl group-containing bisphenol-based epoxy resin (a) having an epoxy equivalent from 150 g/eq to 200 g/eq and a hydroxyl equivalent from 2,000 g/eq to 2,600 g/eq with a polyol compound (b) having an Mn of 200 or more, a polyisocyanate compound (c), and a polyol compound (d) having an Mn of less than 200 serving as a chain extender. The polyurethane-modified epoxy resin uses the epoxy resin (a) in an amount from 20 wt % to 60 wt % with respect to the total amount of the respective components (a), (b), (c), and (d), and contains a polyurethane having the epoxy resin (a) added to both terminals thereof and/or one terminal thereof.