A resonant circuit includes a plurality of cables, each of which including: an outer conductor made of a conductive material in a cylindrical manner; an inner conductor, made of a conductive material in an elongated manner, and disposed inside of the outer conductor; and an insulator disposed between the outer conductor and the inner conductor. The plurality of cables is disposed in series in a circular manner. The inner conductor, provided in one of adjacently disposed cables among the plurality of cables, is conductively connected to the outer conductor of another of the adjacently disposed cable.