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 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, wherein the mean value is an amplitude β of a gradient; and (Sa 4 ) multiplying the 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, wherein β = 1 N ⁢ ∑ i = - k k ⁢ ⁢ ∑ j = - k k ⁢ ⁢ ( x ( i = 0 , j = 0 ) - x ( i , j ) ) i is an index corresponding to a first spatial dimension, j is an index corresponding to a second spatial dimension, k is an arbitrarily defined boundary constant greater than or equal to one, x (i, j) corresponds to the pixels of the target region D, target region D is bounded in one corner by a pixel corresponding to x (i=−k, j=−k) and in another corner by a pixel corresponding to x (i=k, j=k) , N is the number of pixels in the target region D, x (j=0, j=0) corresponds to the pixel of interest in the target region D, and x (i≠0, j≠0) corresponds to the arbitrarily defined neighboring pixels of target region D; the method further including repeating (Sa 3 ) and (Sa 4 ) with respect to a new target region D and new pixel of interest in the new target region D.