A radio wave transmitted from an oscillation source through a first antenna is received each by a second antenna and a third antenna, and a synchronous detection is performed. Variation in impedance of a subject, i.e., variation in impedance of a human body, causes an effect equivalent to performing amplitude modulation with respect to an unmodulated radio wave, and gain fluctuation of a received radio wave caused due to the existence of the human body is cancelled out by a differential amplifier.
