1. An ultrasonic imaging apparatus comprising: an image data production device for producing image data comprising a plurality of frame image data items, the plurality of frame image data items representing a time-sequential change in a visualized region, after a contrast medium is injected into a subject; a display device for displaying images represented by the image data; a storage device for preserving the image data; an infiltrated field image data production device for producing infiltrated field image data using the image data, the infiltrated field image data representing a background image that renders all fields within the visualization region into which the contrast medium is infiltrated; an infiltration process image data production device for producing infiltration process image data using the image data, the infiltration process image data representing a plurality of image frames showing a process of infiltration of the contrast medium into the visualized region; and a synthesis/output device for one of superimposing and synthesizing each frame image of the plurality of frame images on or with the background image to generate a resultant image, and transferring the resultant image to the display device while sequentially changing the frame images according to the process.