Disclosed is a carbon-fiber-reinforced composite material that is suitable for use as a construction material and exhibits high mechanical strength in harsh usage environments such as low-temperature environments and high-temperature moisture-absorbing environments. Also disclosed are an epoxy resin composition for producing the aforementioned carbon-fiber-rein-forced composite material and a prepreg obtained through the use of said epoxy resin composition. Said epoxy resin composition comprises at least the following constituents, by mass with respect to the total mass of the composition: (A) between 20% and 80% of an epoxy resin having the structure represented by formula (1); and (B) between 10% and 50% of an epoxy resin that has at least two ring structures with four or more members each and also has one amine glycidyl or ether glycidyl directly connected to a ring structure.