A transmitting node produces synchronization data to be inserted into plain text and encrypts the thus generated data into multi-valued data so as to transmit the data. The synchronization data indicates the position of a running key used for encryption. A receiving node decrypts a signal including the synchronization data using the running key and detects the synchronization data from the signal to confirm synchronization of the running key between transmitting and receiving nodes. Then, the receiving node transmits a synchronization confirmation signal to the transmitting node. If the transmitting node does not receive the synchronization confirmation signal, it determines that the synchronization of the running key is shifted, and re-synchronization is performed. To perform re-synchronization, a running key ahead of the position of the running key associated with synchronization data that has been stored is generated.