A conductor is mounted on a circuit board parallel to its side along which a radiation of a radio frequency signal is generated. The proximal end of the L-shaped conductor is electrically connected to a ground pattern formed on the rear surface of a circuit board 1, and the distal end of the L-shaped conductor is open. The position at which the conductor is connected to the ground pattern is set to be a position spaced apart by a quarter-wavelength of a radio-frequency signal from a feed point of an antenna. The total length of the conductor is set to be a half-wavelength of the radio-frequency signal.