A radio frequency signal oscillator and an intermediate frequency signal oscillator are incorporated in a communication processing device, and a carrier wave with which an intermediate frequency signal is overlapped is generated using the oscillators and transmitted from an antenna. Mixers separate and extract an I signal and a Q signal from the intermediate frequency signal in the signal received by the antenna. A phase difference detector detects a phase difference of the intermediate frequency signal in a reflected wave to the intermediate frequency signal in the carrier wave using the I signal and the Q signal after the reception of the reflected wave from a tag is started. A distance calculator calculates a distance from the antenna to the tag using the phase difference and a wavelength of the intermediate frequency signal.