A first communication device determines whether an encryption key is stored in a first memory in association with an identifier of data requested to transmit, and transmits the identifier of data requested to transmit and the encryption key associated therewith to a second communication device, when it is determined that the encryption key is stored in the first memory in association with the identifier of data requested to transmit. The second communication device determines whether encrypted data is stored in a second memory in association with the identifier contained in data received from the first communication device, decodes, when it is determined that the encrypted data is stored in the second memory in association with the identifier contained in the received data, the encrypted data with the encryption key contained in the received data, and outputs the decoded data.