In an image processing technology, an original image is inputted by accepting an input of an original image to be image-processed; a local area is set, with a pixel of which brightness is to be converted being a center of the local area, from the original image; a local histogram relating to brightness of the local area is calculated; a local cumulative histogram is calculated by accumulating the local histogram; first and second monotone increasing functions are determined, respectively, corresponding to cumulative frequency values in first and second classes of the local cumulative histogram; first and second weighting functions are determined, respectively, corresponding to the first and second monotone increasing functions; a conversion function relating to brightness of the pixel is produced from the first monotone increasing function weighted by the first weighting function and the second monotone increasing function weighted by the second weighting function; and the brightness of the pixel is then converted by the conversion function.