A mixer is connected to a signal generator and an antenna and outputs a signal at an intermediate frequency. A PLL demodulator subjects the signal at the intermediate frequency from the mixer to PLL demodulation. An amplifier amplifies a signal from the PLL demodulator. A detector detects an amount of shift occurring in the PLL demodulator. A detector detects a gain of the amplifier. An FSK demodulator subjects a signal from the amplifier to FSK demodulation. An AFC unit detects a frequency offset in the signal from the amplifier and causes the signal generator to make a correction for the frequency offset detected.