An antenna array part constituting a communication antenna is composed of a plurality of antenna elements. The antenna elements are arranged on an arc. This allows radio waves to be irradiated also in the exact transverse direction of the antenna array part, thereby making available the communications with an RFID tag positioned in the exact transverse direction. By shifting the phase of a radio frequency signal supplied to each antenna element or adjusting the amplitude of the radio frequency signal, it is possible to change the direction of radio beams irradiated from the antenna array part or expand the width of the radio beams thus making available with RFID tags present in a wider area.