1. An ultrasonic diagnostic apparatus comprising: an ultrasonic probe; a transmitting/receiving device for driving said ultrasonic probe to scan the inside of a subject in a planar manner with an ultrasonic beam; an image producing device for producing an image based on received data obtained from one scan plane; a correlation value calculating device for calculating a correlation value between images; a monitoring scan control device for repeatedly conducting a scan with an ultrasonic beam at such a level as not to break contrast agent until a correlation value between an initial image and a current image becomes smaller than a threshold; and an imaging scan control device for, when the correlation value becomes smaller than the threshold, capturing one image with an ultrasonic beam at such a level as to break contrast agent and returning control to said monitoring scan control device.