1. An ultrasound diagnostic apparatus which outputs an ultrasound diagnostic image of a subject to which an ultrasound contrast agent has been administered, the ultrasound diagnostic apparatus comprising: an ultrasound probe which transmits ultrasound to the subject and which receives echo signals reflected back from the subject; imaging means for forming images from fundamental components of the echo signals received by the ultrasound probe from the subject and for forming images from harmonic components of the echo signals received by the ultrasound probe from the subject, any one of the images corresponding to any one of the echo signals received from the subject; motion detection region setting means for selecting a first image from among the images formed from the fundamental components of the echo signals, selecting a second image that is different from the first image from among the images formed from the fundamental components of the echo signals, and setting, as a motion detection region, a region of a portion of the first image and a portion of the second image; detection means for detecting an amount of movement between the first image and the second image in the motion detection region; and correction means for performing, based on the amount of movement detected, a position adjustment of a third image formed from the harmonic components of the same echo signal from which the first image was formed, such that a position of the third image is corrected to match a position of the second image.