An antenna coil 10 comprises an air-core coil 12 which is wound helically in a plane and a plate magnetic core member 13 which is inserted in the air-core coil 12 so as to be approximately parallel with a plane of the air-core coil 12. The magnetic core member 13 is formed by a soft magnetic metal, an amorphous or ferrite, or a composite member of a powder, flake and plastic, or rubber. The magnetic core member 13 is formed by performing an injection molding operation or a compressing molding operation of the composite member. Alternatively, the magnetic core member 13 is a magnetic coating which is formed by applying and drying the composite member. A non-magnetic conductive plate 14 which has a conductivity is layered on a surface of the air-core coil 12 through which the magnetic core member 13 is inserted. The conductive plate 14 is made of a copper, a copper alloy, an aluminum or an aluminum alloy having 0.01 to 2 mm thickness. By doing this, it is operated by relatively high frequency while it is rigid relatively.