When an SIP interface unit of a server apparatus receives an SIP message for call connection from a client apparatus and an SIP message analyzing unit can confirm that the SIP message is normal, a call controller recognizes that an RTP communication is carried out between the client apparatus and another client apparatus and instructs an encrypting capability management unit to determine RTP encrypting information which is used between the client apparatuses. The encrypting capability management unit determines the RTP encrypting information between these client apparatuses based on the instruction. With this arrangement, there can be provided a client-server distributed system that can realize an encrypting security function without requiring a certificate authentification function at a low cost in order to deliver an encrypting key as well as without necessity of holding or managing a certificate and preparing an authenticating server in a system.