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 mutual difference value which is 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 a plurality of images which correspond to one another on a periodic basis from the respective image groups; 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 the base image searching unit sets an image corresponding to a maximum mutual difference value, among mutual difference values of the respective images constituting the image string, as the representative base image, and with use of the representative base image as the starting point, sequentially searches for images which are closest to a position distant by a virtual period, which is set as a temporary period of the motion of the object, from a plurality of images corresponding to local maximum mutual difference values, and sets the images as the plurality of base images; characterized in that the mutual difference value is calculated with the following formula: Mutual Difference Value z = ∑ w = 0 l ∑ x = 0 m ∑ y = 0 n p x, y, z + w × p x, y, z + w − p x, y, z + w + 1 wherein x, y, and z represent coordinate values along the respective axes in an X-Y-Z Cartesian coordinate system, and p represents a pixel value corresponding to each coordinate within the tomographic image data, the Z-axis direction being the direction of displacement between the scanning planes which are formed to be almost parallel to the X-Y plane. 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 mutual difference value which is 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 a plurality of images which correspond to one another on a periodic basis from the respective image groups; 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 the base image searching unit sets an image corresponding to a maximum mutual difference value, among mutual difference values of the respective images constituting the image string, as the representative base image, and with use of the representative base image as the starting point, sequentially searches for images which are closest to a position distant by a virtual period, which is set as a temporary period of the motion of the object, from a plurality of images corresponding to local maximum mutual difference values, and sets the images as the plurality of base images; the mutual difference value is calculated with the following formula: Mutual Difference Value z = ∑ w = 0 l ∑ x = 0 m ∑ y = 0 n p x, y, z + w × p x, y, z + w − p x, y, z + w + 1 wherein x, y, and z represent coordinate values along the respective axes in an X-Y-Z Cartesian coordinate system, and p represents a pixel value corresponding to each coordinate within the tomographic image data, the Z-axis direction being the direction of displacement between the scanning planes which are formed to be almost parallel to the X-Y plane.