A server receives an input image group from an MFP. The server decides a reference image from the input image group, and generates rotated images by deforming the reference image. The server calculates image feature amounts from the reference image and the rotated images, and generates direction determiners using the image feature amounts. With respect to each of images of the input image group, the server determines the directions of the images by applying the direction determiners, and when the directions of the images are different from the direction of the reference image, the images are rotated to the direction of the reference image, and the directions of each of the images of the input image group are unified. The server generates a classification rule to be used in classification of images using the input image group in which the directions are unified.