An exchange system comprising: an exchange server module which is adapted to connect exchange-connected terminals to each other; pseudo exchange-connected terminal modules which are adapted to be associated respectively with external IM terminals and which are adapted to function as exchange-connected terminals with respect to the exchange server module; pseudo IM terminal modules which are adapted to be associated respectively with the exchange-connected terminals and which are adapted to function as external IM terminals with respect to an external IM server; a database which is adapted to retain association relations between first user identification information to be used in the exchange-connected terminals and second user identification information to be used in the external IM terminals; a pseudo IM terminal & pseudo exchange-connected terminal module which is adapted to retrieve the second user identification associated with the first user identification information from the database, retrieve the first user identification associated with the second user identification information from the database, function as the exchange-connected terminal with respect to the exchange server module by using the first user identification information, and cause one of the pseudo IM terminal modules to function as an associated external IM terminal with respect to the external IM server by using the second user identification information, and means for synchronizing a personal address book used by individual one of the exchanged-connected terminals with an external address book used by the external IM terminals making their contents the same, and for synchronizing a shared address book used by all of the exchange-connected terminals with the external address book without altering the contents of the shared address book. an exchange server module which is adapted to connect exchange-connected terminals to each other; pseudo exchange-connected terminal modules which are adapted to be associated respectively with external IM terminals and which are adapted to function as exchange-connected terminals with respect to the exchange server module; pseudo IM terminal modules which are adapted to be associated respectively with the exchange-connected terminals and which are adapted to function as external IM terminals with respect to an external IM server; a database which is adapted to retain association relations between first user identification information to be used in the exchange-connected terminals and second user identification information to be used in the external IM terminals; a pseudo IM terminal & pseudo exchange-connected terminal module which is adapted to retrieve the second user identification associated with the first user identification information from the database, retrieve the first user identification associated with the second user identification information from the database, function as the exchange-connected terminal with respect to the exchange server module by using the first user identification information, and cause one of the pseudo IM terminal modules to function as an associated external IM terminal with respect to the external IM server by using the second user identification information, and means for synchronizing a personal address book used by individual one of the exchanged-connected terminals with an external address book used by the external IM terminals making their contents the same, and for synchronizing a shared address book used by all of the exchange-connected terminals with the external address book without altering the contents of the shared address book.