PURPOSE: To improve an S/N ratio through regulation of resolution to a proper value by a method wherein a mixing output of a reflection echo signal from a Doppler signal processing system and a reference wave signal is integrated for a given period, and an integrating time is controlled.

CONSTITUTION: In a Doppler signal processing system 4, an echo signal generated by a transmitting receiving wave circuit 2, a reference wave signal from an oscillator 3, and a signal in which the reference wave signal is phased in a 90° arc are mixed by means of mixers 6 and 7 to detect a quadrature. The detection output is inputted to A/D converting circuits 8 and 9 after a harmonic component is cut. An output from an A/D converting circuit 8 is inputted, in order, to data resistors 20. A gate circuit 21 enables setting of an integrating time by means of an enable signal, and the enable signal is set by a gate control circuit 25 based on data from a control part 24. As noted above, a signal from each gate circuit 21 is added by an adder 22 and a result is inputted to an average value circuit 23. In the average value circuit 23, by dividing an addition output with an integrating time, an average value is computed, and an output therefrom is outputted to an MTI filter 12.