PURPOSE: To correct a ciphering key number at a receiver side even when the ciphering key number is not correctly reported to the receiver side with respect to the ciphering communication system having plural ciphering keys used to cipher data and using one of them switchingly at any time.

CONSTITUTION: A transmitter 100 is provided with additional data addition means 101 adding any additional data to data to be ciphered, and a receiver 200 is provided with an additional data detection means 202 detecting the additional data from the data after decoding and a ciphering key discrimination means 203 revising sequentially the ciphering key of the receiver 200 when the additional data cannot normally be detected. Thus, even when the number of a ciphering key is not normally reported on the revision of a ciphering key, it is recovered by the receiver side only.