Provided is a system for delivering an instant message in which instant messaging systems of different types interwork. The system includes a transmitter mobile communication terminal and an interworking server. The transmitter mobile communication terminal connected to a first instant messaging system transmits a request for an instant message receipt report to a receiver mobile communication terminal connected to a second instant messaging system. The interworking server converts the delivery format of the instant message into another suitable for the second instant messaging system and transmits the instant message to the receiver mobile communication terminal. The interworking server also converts the delivery format of the instant message receipt report generated according to the receipt result of the instant message by the receiver mobile communication terminal after completion of the delivery of the instant message to the receiver mobile communication terminal into another suitable for the delivery format of the first instant messaging system to which the transmitter mobile communication terminal is connected, and transmits the instant message receipt report to the transmitter mobile communication terminal.