The sense of contrast perceived by a viewer of an image is quantified and adequate image processing is carried out on image data based on the sense of contrast. Contrast-sense quantification means generates unsharp image data of the image data and then generates a histogram of the unsharp image data. Since the histogram of the image data includes lightness information of details of the image, a distribution width thereof does not represent the contrast perceived by the viewer of the image as a whole. However, since the histogram of the unsharp image data excludes information of the details, a distribution width of the unsharp image data represents the contrast of the overall image. The distribution width of the histogram of the unsharp image data is found as the sense of contrast and input to processing means. In the processing means, tone conversion processing is carried out on the image data by changing a tone conversion LUT based on the sense of contrast, and processed image data are obtained.