An antenna coil is formed by a wire rod obtained by combining and integrating two or more kinds of materials having different magnetisms. The wire rod has a circular or polygonal cross sectional shape. The two or more kinds of materials having different magnetisms are combined so that the magnetisms of the combined materials are set off. The wire rod is wound around a bobbin so as to have a solenoid shape. Desirably, the low-magnetic wire rod is placed in an atmosphere whose temperature has been reduced to 10.degree. K or less or superconductive filaments are formed in the outermost layer. Preferably, a part of the superconductive filaments are exposed.