An image processing method is performed by a processor of an image processing apparatus which performs image processing of an input frame image. In the image processing method, a tonal histogram is generated, of which a gross area corresponds to a total number of pixels in the input frame image, and contrast is calculated from the tonal histogram. An input of a motion-detecting level with respect to the input frame image is received. A motion-detecting threshold corresponding to the motion-detecting level and the contrast is calculated. A composite image of the input frame image and a preceding frame image of the input frame image is generated based on a comparison result of a finite difference value between the input frame image and the preceding frame image, and the motion-detecting threshold.