In a learning process, first, images having different resolutions are obtained from a target region of the subject (S101). Further, the subject characteristic of the target region is obtained (S102). Then, the resolution conversion rules are learned from the images having different resolutions, and those are recorded to a storage device along with the subject characteristics (S103). When converting the resolutions, the resolution conversion rules learned for the corresponding subject characteristics are applied to each region of the original image so as to convert the resolutions of the original image.