Abstract: To provide a communication system which enables to mutually find out a presence status of a communication counterpart in real time between users of an instant message service and enables to transmit and receive a message to and from the communication counterpart who uses a different message type. An IPM server converts a message type of an instant message transmitted from an SMS client, an MMS client, or an IPM client so as to match that of a destination terminal, acquires presence status information indicating whether or not the clients are communicable, and notifies each of the clients of the acquired presence status information.