An antenna coil includes an air-core coil wound helically in a plane and a plate magnetic core member inserted in the air-core coil to be approximately parallel with a plane of the air-core coil. The magnetic core member 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 is formed by performing an injection molding operation or a compressing molding operation of the composite member. Alternatively, the magnetic core member is a magnetic coating formed by applying and drying the composite member. A non-magnetic conductive plate that has a conductivity is layered on a surface of the air-core coil through which the magnetic core member is inserted. The conductive plate is made of a copper, a copper alloy, an aluminum or an aluminum alloy having 0.01 to 2 mm thickness. The antenna coil is operated by relatively high frequency while it is rigid relatively.