Resin composition of the present disclosure includes: an epoxy resin; and a hardener, in which the hardener contains a styrene-maleic anhydride copolymer (SMA) and an anhydride having only one anhydride group in a molecule. An acid value of the SMA is in a range from 300 to 550, inclusive. A ratio of an anhydride equivalent number of the anhydride with respect to an epoxy group equivalent number of the epoxy resin is in a range from 0.05 to 0.5, inclusive. A ratio of the total number of the equivalent numbers of the anhydride groups of the anhydride and the SMA with respect to the epoxy group equivalent number is a range from 0.5 to 1.2, inclusive.