JPEG-2000 data is formatted into RTP packets for transmission through the Internet. A priority field is provided in the format, and a value indicating the importance of a code stream included in a packet is stored therein. A type field indicates a payload type of a JPEG-2000 code stream. An mh_id field is provided as an identifier of a main header of the JPEG-2000 code stream and indicates a change in the main header. An mh_length field indicates the length of the main header and a fragment offset field stores the number of offset bytes from the start of an image of the packet.