PURPOSE: To omit the setting of an expensive extension calling state display board or the like by returning an exchange system to a normal extension mutual connection regulating state automatically when temporally permitted extension mutual connection is repeated by a previously fixed number of times.

CONSTITUTION: If a non-lock switch is operated when the exchange system is in the extension mutual connection regulating state, said state is detected by the operation of an interface device 2, a central control device 3 and an exchange processing program memory 4 and the system is turned to the extension mutual connection released state. If the extensions are mutually connected under said state, a counter is counted up by +1 and the contents of the counter are compared with the previously fixed number of times of permission of extension mutual connection. When both values coincide with each other, the system is returned to the extension mutual connection regulating state again. Since the method makes it possible to permit the connection only by the fixed number of times by depressing the switch, an expensive extension calling state display board or operation returning the switch to the original state which is necessary for starting the regulation again can be omitted.