PURPOSE: To measure Doppler frequency accurately.

CONSTITUTION: A demodulator 17 demodulates a reception data from a received signal and at the same time outputs a voltage which changes according to the frequency of the received signal as a VCO voltage, an A/D converter 18 converts the VCO voltage from the demodulator 17 to a digital signal, and then a central processing unit 19 calculates the frequency of the received signal based on the VCO voltage. The central processing unit 19 outputs a frequency control signal for controlling the oscillation frequency of a voltage-controlled oscillator 14 via a D/A converter 20 and at the same time adds the frequency of the received signal and the oscillation frequency which is estimated from the control voltage to the voltage-controlled oscillator based on the VCO voltage, thus measuring the Doppler frequency and hence providing a reliable and compact title device without any adjustment.