1. An ultrasound diagnostic apparatus, comprising: a unit which transmits and receives ultrasonic waves with respect to a three-dimensional space within a living organism to obtain original volume data; a computation unit which extracts motion information from the original volume data to generate motion volume data; an identification unit which specifies a plurality of objects included in the motion volume data and identifies a type of each of the objects which are specified to thereby generate, as identification data, three-dimensional mask data for identifying a noise portion in the motion volume data; an extraction unit which extracts bloodstream volume data from the motion volume data, the extraction unit having a three-dimensional gate-processing unit which removes the noise portion in the motion volume data by performing a logical operation between the three-dimensional mask data and the motion volume data; and an image-forming unit which forms a three-dimensional bloodstream image by reference to the bloodstream volume data.