1. An ultrasonic diagnostic apparatus comprising: an ultrasonic probe; a transmitting/receiving device configured to drive said ultrasonic probe to scan a subject in a planar manner with an ultrasonic beam; a low-mechanical index (MI) scan control device configured to repeatedly conduct a scan with the ultrasonic beam at a level as not to break a contrast agent to obtain an initial image and a current image; an image producing device configured to produce a plurality of images, each image of the plurality of images based on received data obtained from one scan plane, the plurality of images including the initial image, the current image, and a contrast-enhanced image; a correlation value calculating device configured to calculate a correlation value between the initial image and the current image; and a high-MI scan control device configured to when the correlation value is less than a threshold value to capture the contrast-enhanced image with the ultrasonic beam at a level to break the contrast agent and also to return control to said low-MI scan control device.