To properly perform frequency shift to Doppler information before removing a clutter component from the Doppler information by using MTI (moving target indicating) filters.

A shift frequency calculating part calculates a shift frequency from a central frequency and a variance of the Doppler information. In such a case, when the variance is great, the shift frequency is calculated while taking into account that the clutter component is not dominant so much. A frequency shift circuit 20 carries out the frequency shift to the Doppler information, and the clutter component is then removed by the MTI filters 22 and 24. A flow velocity correcting circuit 28 carries out frequency shift inverse to the frequency shift to a flow velocity value resulting from autocorrelation.