1. An ultrasound diagnostic apparatus comprising: an ultrasound probe which has ultrasound transducers transmitting ultrasonic waves, receiving an ultrasonic echo reflected by a subject, and outputting a reception signal according to the received ultrasonic echo; an image producer which produces a B-mode image and an M-mode image from the reception signal output from the ultrasound transducers; and a boundary detector which detects a blood vessel anterior wall boundary in the B-mode image using a time phase corresponding to the B-mode image of the M-mode image.