An image processor includes a JPEG 2000 codec. The JPEG 2000 codec generates a plurality of levels of encoded data, and a checksum circuit integrates the encoded data for each of the levels to determine a checksum. The determined checksum is written together with the encoded data into a header of a stream. In decoding a desired level of the encoded data, a checksum is determined again on the basis of the encoded data and compared with the checksum written in the header. Accordingly, if the two matches with each other, it is concluded that the encoded data is not tampered. If the two does not match with each other, it is concluded that the encoded data is tampered and a warning message is displayed on a monitor.