1. An image processing method for adjusting a size of an image signal of each pixel included in an input image by a computer according to a predetermined purpose, the image processing method comprising the steps of: (Sa 1 ) inputting data of an input image into a computer; (Sa 2 ) inputting an arbitrarily defined constant m as a contrast intensity; (Sa 3 ) demarcating a closed region (x, y) made up of a plurality of pixels in the input image as a target region D, summing degrees of inclination between respective two pixels, namely, a pixel of interest in the target region D and arbitrarily defined neighboring pixels thereof, over a whole circumference around the pixel of interest, and dividing the sum by the number of pixels within the target region to calculate a mean value thereof; and (Sa 4 ) multiplying an amplitude value β of a gradient obtained in step (Sa 3 ) by the contrast intensity m (where m is a positive constant) inputted in step (Sa 2 ) and adding an arbitrarily defined offset value γ to the obtained value.