To highly accurately measure Doppler frequency by compensating a peak frequency of a base band with a base frequency of an image band specified by rough Doppler frequency.

A DSP 16 accepts echo (pulse) signals which are transmitted and received by a transducer 1 and sampled by an A/D-converter 15 with constant intervals (PRF) and measures a Doppler frequency of a single pulse mode for obtaining a rough Doppler frequency. Then original pulse signal frequency components other than a base band spectrum are further removed to make data into a complex number. A predetermined period at the center of the signals is averaged to obtain a PRF sampling data, and it is Fourier-transformed to a base band with a number of its newest predetermined points for detecting a peak frequency. Then an image band where a true Doppler frequency exists is determined by the rough Doppler frequency with a required number of times averaged to obtain accuracy of PRF or lower, and a base frequency of this band, n×PRF, is added to the peak frequency to calculate a precise Doppler frequency.