In order to encrypt a part of an input image in such a manner that the encrypted part can be located for decryption, means for regularly converting pixel values in the area to be encrypted and generating a pattern unique to the pixel value conversion in order to encrypt the area to be encrypted; means for setting a specified positioning marker for locating an encryption position at two or more of the four corners of the area to be encrypted; and means for setting one or more checking marks for verifying appropriateness of a decrypted image in the encrypted area before the decryption are included.