At the time of marker attachment (at the time of encryption), for example, image conversion in which pixels to be overwritten by a marker are selected and pruned in a distributed manner and the pruning positions are moved to a marker attachment position is performed, and the marker is attached to the marker attachment position obtained as a result. At the time of marker elimination (at the time of decryption), for example, image reverse conversion in which each pixel of the marker is moved to the original position before the marker attachment is performed, each moved pixel of the marker is recovered, and the marker is eliminated.