A resonant circuit and an antenna device achieve a low resonance frequency without increasing a coil size, and improve communication performance. In the resonant circuit, two coil-shaped conductors are arranged so as to be opposed to each other with a dielectric sheet interposed therebetween. The two coil-shaped conductors are, at the opposed portions thereof, coupled with a capacitance interposed therebetween, and wound so that electric currents flowing through the respective conductors trend in the same direction in a planar view. The opposed area in at least a portion of the outermost windings and/or innermost windings of the coil-shaped conductors is larger as compared with the opposed area in any other winding, and the respective ends of the conductors define power feeding units.