1. An ultrasound diagnostic apparatus comprising: a probe that emits and receives ultrasonic waves to and from a three-dimensional region including an object in periodic motion; an emission and reception controlling unit that controls the probe such that a scanning plane is displaced over a plurality of periods of the motion so as to form a plurality of scanning planes within the three-dimensional region; a base image searching unit that searches for a plurality of base images from an image string constituted of a plurality of images corresponding to the plurality of scanning planes based on a feature amount relating to the periodicity of the motion; an image reconfiguration unit that divides the image string into a plurality of image groups using the respective base images as dividing units, and extracts from the respective image groups a plurality of images which correspond to one another on a periodic basis; and a display-image-forming unit that forms a display image of the object based on the plurality of images which correspond to one another on a periodic basis, wherein position information for identifying a position of an image in the three-dimensional region is attached to each image obtained from each of the scanning planes, and the display image is formed while each of the plurality of images which correspond to one another on a periodic basis is correlated to a position within a display region identified by respective position information.