Low-resolution image data for recognizing signal components macroscopically is generated, an effective diagnostic component and an ineffective diagnostic component are determined by using the low-resolution image data, and a correction coefficient is decided for every pixel such that the correction coefficient is set to 1 for the effective diagnostic component and the correction coefficient is set to equal to or larger than 0 and smaller than 1 for the ineffective diagnostic component. In addition, correction processing for raising the contrast between the effective diagnostic component and the ineffective diagnostic component by multiplying each pixel value of original image data by a corresponding correction coefficient is executed.