It takes time for an encryption data communication system to transfer encrypted data, because negotiations of security parameters are necessary prior to communications in order to protect security and integrity of a SIP message or public key cryptography is required to be used for an encryption process, a decryption process., an digital signature process and an digital digital signature verification process each time a SIP message is transmitted/received. When a SIP message is transferred between two entities, the message is encrypted by shared information if the information is being shared between the entities, or the message is encrypted by the public key of the transmission destination entity if the shared information is not being shared. The encrypted message contains shared information to be used for the transmission destination entity of the encrypted data to encrypt or decrypt the message, during communications after the encrypted data is generated.