A region where a detecting target object exists is extracted by a comparison between an evaluated value indicating a probability that the detecting target object exists and a threshold, through the process of producing a differential image between different frames in plural frames constituting a continuous image, of setting an average value in an averaging region extended around each pixel of the differential image as a new value of each pixel, of obtaining the evaluated value by applying a filter that acts on a search region on an image to a search region extended around a search pixel that is extracted by comparing the new value and the threshold, on the differential image.