When an error is detected in a received header, in estimating reference information while assuming an error in a packet receiving interval, a header is decompressed using at least one value of another candidate sequence numbers used in correcting an erroneous sequence number, corresponding to a time that elapses between previously receiving a packet correctly and receiving a current packet and to the packet receiving internal. It is thereby possible to increase a possibility of estimating the reference information correctly and suppress the number of discarded packets at a receiving side, while suppressing increases in introduced processing amount in data transmission with header compression.