A technology for performing the filter processing of blood flow velocity data having aliasing by only common filter processing means and common addition and subtraction means and actualizing image display of smooth blood flow velocity is disclosed. According to this technology, when performing image display of the blood flow velocity of blood within a subject biological body using an ultrasonic wave Doppler method, the difference values of data D33 of an arbitrary focus point and data D32 and D34 of points spatially prior and subsequent to this focus point are calculated in the pre-processing unit (difference calculating unit) 41. Then, in the filtering unit 42, after filter processing is performed on these difference values, data D39 after filter processing and data D33 of the arbitrary focus point are added in the post-processing unit (addition processing unit) 43. In this way, a filter processing can be performed under the assumption that the velocity difference is smaller than the velocity difference generated by the aliasing phenomenon, and filter processing of the blood flow velocity value in the sound ray direction of the ultrasonic waves can be performed without being influenced by aliasing.