A resonant circuit having a circuit coil formed on each surface of a dielectric film. Each coil has a line-width and an outer periphery in which the line-width of each circuit is 1/50 or more of the length of the outermost periphery of that circuit. Each coil has at least three complete turns, and the coils are provided on the faces of a dielectric film in such a manner that the coils are faced to each other.