A transmitter and a receiver each have an apparatus for carrying out retransmission processing at a TCP layer and an apparatus for carrying out encrypted-data communication at an IPSec layer. The apparatus operating at the IPSec layer has a buffer for storing encrypted data transmitted by the transmitter. When a request for retransmission of data is received from the receiver, in spite of an operation carried out at an IP layer, information available at the TCP layer is obtained and data corresponding to the encrypted data stored in the buffer is returned to the receiver in response to the request.