To track the movement of a common subject contained in a sequence of image frames time-sequentially captured by a digital camera, feature points of the subject are extracted from a first image frame, and corresponding points to the extracted feature point are identified in a second image frame that is captured after the first image frame. When the number of traceable feature points, to which the corresponding points can be identified, is less than a predetermined value, the speed of movement of the subject is judged high, and the shutter speed of the digital camera is set at a higher value.