PURPOSE: To simplify in interface between exchange units by grasping a line when a subscriber in a supervisory source unit is idle and requiring the connection of a subscriber to be supervised.

CONSTITUTION: When a supervisory request subscriber SUBa registers the idle supervisory request of a subscriber to be supervised SUBa, a supervisory unit Ua periodically supervises whether the subscriber SUBa is idle. When the subscriber SUBa becomes idle, it is made in a grasped state. Then the unit Ua transmits a connection request to a unit Ub to be supervised. If the subscriber SUBa is idle, the unit Ub connects a trunk TRKb to the subscriber SUBb, and reports the completion to the unit Ua. It calls the subscriber SUBa, transmits the call request of the subscriber SUBb after the subscriber SUBa responds the call.