1. A motion detection method comprising the steps of: digitizing a background differential image corresponding to a differential image between an input image sequentially input for each frame and a preset background image so as to generate a digitized image composed of a target region different from the background image and a background region with no difference from the background image for each frame; comparing the digitized images between frames to extract an overlap region remaining as the target region, an increased region changed from the background region to the target region, and a decreased region changed from the target region to the background region; obtaining region parameters composed of the number of pixels contained in the region, a sum of X-coordinates obtained by adding X-coordinate values in two-dimensional coordinates indicating a position of a pixel for all the pixels contained in the region, and a sum of Y-coordinates obtained by adding Y-coordinate values for all the pixels contained in the region, for each of an integrated region, the increased region, and the decreased region, the integrated region being obtained by integrating the overlap region, and the increased region and the decreased region adjacent to the overlap region; and detecting a motion of a moving object displayed on the integrated region for each of the integrated regions based on the region parameters.