An FSK modulator 20 applies FSK modulation processing to an RF wave supplied from an RF wave oscillator 22, by means of frequency shift keying (FSK) based on a periodical signal sequence supplied from a pattern generator 24, to thereby generate an FSK continuous wave. The continuous wave output from the FSK modulator 20 is delayed in delay circuits 26I and 26Q and is then supplied, as a reference signal, to each of mixers of a receiving mixer 30. Each of the delay circuits 26I and 26Q delays the continuous wave by a delay amount in accordance with a depth of a target position and outputs a delayed reference signal. Thus, demodulation processing is performed with a correlation between a reception signal from the target position and the reference signal being enhanced, and Doppler information from the target position is selectively extracted by a Doppler information analyzing unit 44.