Provided is a polyurethane-modified epoxy resin composition having satisfactory operability of processing, such as casting or impregnation, in a composition state. The epoxy resin composition includes, as essential components, the following components (A) to (D): (A) a low-concentration polyurethane-modified epoxy resin containing a polyurethane having an epoxy resin added to each of both terminals thereof and/or one terminal thereof; (B) a polyurethane-unmodified epoxy resin that is liquid at 30° C.; (C) a solid epoxy resin having a bisphenol structure, the resin having a glass transition temperature or a melting point of 50° C. or more; and (D) an amine-based curing agent that is dicyandiamide or a derivative thereof, wherein the epoxy resin composition includes the component (A) at from 3.0 wt % to 30.0 wt %, and the component (C) at from 0.1 wt % to 40.0 wt % with respect to the total of the components (A) to (D).