Heretofore, an operator has been required to judge the type of image on a display and select any of various effect processings manually, thus automation has been impossible. By totaling luminance values of pixels into a luminance distribution which pixels have been selected by a thinning-out processing for image data (step S102) it becomes possible to count the number of colors used (step S104). As a result, if the number of colors used in image data is large, it can be judged that the image data is of a natural picture. Besides, a contrast enlarging process (step S110), a saturation highlighting process (step S112) and an edge highlighting process (S114), which are suitable for application to a natural picture on the basis of the results of such judgment, can be selected automatically while changing the respective highlighting degrees. Also in a printing process, if an inputted image is a natural picture, there is performed a color transformation using a pre-gray level transformation (S508), while if the image is not a natural picture, there is performed a color transformation using a combination of cache+interpolating operation (S506), and thus it is possible to automatically select a color transformation processing of a small processing volume.