The present invention relates to an ultrasound diagnostic apparatus which forms a three-dimensional bloodstream image by reference to volume data obtained from a three-dimensional space within a living organism. Binarization processing and three-dimensional labeling processing are applied to velocity volume data, to thereby generate three-dimensional mask data. At this time, because a bloodstream object has a larger volume size than a noise object, this difference in volume size is utilized to discriminate between a bloodstream portion and a noise portion. Bloodstream volume data are then generated from the velocity volume data and by reference to the three-dimensional mask data. Then, a three-dimensional bloodstream image is formed by reference to the bloodstream volume data.