A method for transferring a session between clients is provided when one user uses multiple clients on a service (for example, Push to talk over Cellular (PoC), Instant Messaging (IM), Converged-Internet Protocol (IP) Messaging (CPM)) providing system implemented through a Session Initiation Protocol (SIP)/IP core network. A session transfer method is provided when the user needs to transfer a session to another client while the session is performed through one client. A “temporary storing” technique is used in which a conversation server or client as one of elements of a seamless CPM system for providing a seamless user environment temporarily stores data transmitted from another user during session transfer.