A velocity vector calculating unit 40 obtains velocity vectors at various coordinates in a blood flow, on the basis of signals obtained by transmitting and receiving ultrasonic waves, thereby generating a plurality of vector frames, each formed from velocity vectors at a plurality of coordinates. An interpolation processing unit 50 generates interpolated frames by employing an interpolation process between two adjacent vector frames, and adds one or a plurality of such interpolated frames between said vector frames. The interpolation processing unit 50 generates each interpolated frame, formed from interpolated vectors at a plurality of coordinates, by means of an interpolation process based on velocity vectors between the two adjacent vector frames.