In a confidential-communication system that uses a first-communication network that is Internet capable of confidential communication using VPN, and a second communication network that is an audio-circuit network, a method is implemented wherein a send/receive-processing portion 5 of a communication control device 3 completes authentication between users by implementing a P2P connection between each communication control device 3 by referencing specific information that specifies another party of a P2P connection in a memory portion 4 on the communication device 3 before confidential communication starts using VPN; a confidential-communication preparation portion 70 of the communication control device 3 exchanges via the second communication network VPN joint information required to establish a VPN link with the first communication network; and a switching portion 71 of the communication control device 3 starts confidential communication using VPN over a first-communication network.