An image processing apparatus comprises a luminance extraction means for extracting a luminance component from image data, a scale conversion means for obtaining a distribution of the luminance component on a relatively large scale, a luminance improvement means for improving the luminance distribution of the image data by using the luminance component and an output from the scale conversion means, and an image reproduction means for reproducing the image data by using an output from the luminance improvement means as a luminance distribution of a new image, wherein the luminance improvement means further comprises a parameter adjustment means for adjusting a degree of improvement by means of a parameter. Thus, a brightness distribution is optimally improved according to image quality of a digital image to be processed.