The invention provides a communication system, which reduces load such as conversation management and relay treatment, etc. of a SOAP (Simple Object Access Protocol)-SIP (Session Initiation Protocol) adapter providing 3PCC (3rd Party Call Control) services. The SOAP-SIP adapter (2) receives connection request messages of a first terminal (5b) and a second terminal (5c) from a web server (1) and then respectively establishes a conversation with the first terminal (5b) and the second terminal (5c). After the conversation establishment with the frist terminal (5b) and the second terminal (5c), the adapter (2) sends converstion transmission requests (Refer) to a call control device (30) which then manages the conversation between the first terminal (5b) and the second terminal (5c) which established a conversation with the adapter (2).