PURPOSE: To use a specific subscriber relay installation efficiently by identifying a dependent incoming trunk group with respect to an incoming call.

CONSTITUTION: A trunk group number is allocated to an incoming trunk group 2 and an outgoing trunk group 4 and both the trunk group numbers with respect to relay connection control via a network 5 to an incoming call are used by a relay connection control means 6. When the control means 6 discriminates it to be a relay call, an outgoing trunk group decision means 14 in a relay route uses a number of an incoming trunk group to which an incoming call belongs to retrieve a group attribute table 8, a route decision table 10 and an outgoing trunk group decision table 12 in a relay route. The outgoing trunk group number with respect to the relay call is decided by the retrieval and the group number is used for usual relay connection control by the control means 6.