A method for managing a message thread including a series of messages transmitted to and received from a same counterpart by an Inter-Working Function, IWF, that interworks a Converged Internet Protocol Messaging Service, CPMS, and another messaging service, the method comprising the steps of: receiving, from a first client terminal that supports a Session Initiation Protocol, SIP, based CPMS, a SIP message for a conversational message communication, in which a second client terminal supporting another messaging service and not the CPMS is appointed as a recipient terminal; detecting a thread Identification, ID, an address of the first client terminal, and an address of the second client terminal from the SIP message; storing the thread ID, the address of the first client terminal, and the address of the second client terminal in a database; converting a message format of the SIP message into a message format that is supported in the other messaging service; and transmitting the converted SIP message to the second client terminal; wherein the method further comprises: receiving a second message, which targets the first client terminal, from the second client terminal; converting a message format of the received second message into a message format that is supported in the CPMS to include the thread ID detected from the database; and transmitting the converted second message including the thread ID detected from the database to the first client terminal, wherein upon receiving the SIP message transmitted from the first client terminal, a valid reply period corresponding to the thread ID is reset; and wherein upon receiving the second message transmitted from the second client terminal, the valid reply period corresponding to the thread ID is reset, wherein the thread ID, the address of the first client terminal and the address of the second client terminal are deleted from the database, if the IWF fails to receive the SIP message transmitted from the first client terminal to the second client terminal or the second message transmitted from the second client terminal to the first client terminal within the valid reply period. receiving, from a first client terminal that supports a Session Initiation Protocol, SIP, based CPMS, a SIP message for a conversational message communication, in which a second client terminal supporting another messaging service and not the CPMS is appointed as a recipient terminal; detecting a thread Identification, ID, an address of the first client terminal, and an address of the second client terminal from the SIP message; storing the thread ID, the address of the first client terminal, and the address of the second client terminal in a database; converting a message format of the SIP message into a message format that is supported in the other messaging service; and transmitting the converted SIP message to the second client terminal; receiving a second message, which targets the first client terminal, from the second client terminal; converting a message format of the received second message into a message format that is supported in the CPMS to include the thread ID detected from the database; and transmitting the converted second message including the thread ID detected from the database to the first client terminal, wherein upon receiving the SIP message transmitted from the first client terminal, a valid reply period corresponding to the thread ID is reset; and wherein upon receiving the second message transmitted from the second client terminal, the valid reply period corresponding to the thread ID is reset, wherein the thread ID, the address of the first client terminal and the address of the second client terminal are deleted from the database, if the IWF fails to receive the SIP message transmitted from the first client terminal to the second client terminal or the second message transmitted from the second client terminal to the first client terminal within the valid reply period.