Floor negotiation method for a poc session

A floor negotiation method for a PoC session is provided to supply equal and similar user experiences to a PoC v2.0 client in which existence possibility of the PoC box can be predicted without a PoC v1.0 client not supporting a PoC box and recording notification. When a session initiating PoC client(10) intends to give up an initial floor, a session inviting message having the first parameter indicating the intention is transmitted to the first PoC server(20). When the first PoC server receives the session inviting message including the first parameter, a session inviting message including the second parameter is transmitted to the second PoC server(30). The second parameter indicates suggestion of occupying a floor to another client. When the second PoC server receives a session inviting message including the first parameter, the session inviting message is transmitted to another PoC client(40).