A method for performing session merging between PoC sessions under the environment of multiple PoC sessions is provided. A merging PoC client sends a request for the session merging with a merged session to a PoC server managing a merging session to which the PoC client belongs. The PoC server managing the merging session sends a session merge request to a PoC server managing the merged session. Negotiation of a CF (Controlling PoC Function) for managing a session to be merged is performed between the PoC server of the merging session and the PoC server of the merged session. When an entire CF of the session to be merged is selected as the negotiation between the PoC servers is completed, the PoC server of the merged session sends an UPDATE message of modified media parameters to a PoC client of the merged session. The PoC server of the merging session sends, to a PoC client of the merging session, a NOTIFY message of a merge request notifying that the PoC server of the merging session is selected as the entire CF.