PURPOSE: To provide the method to hardly generate a congested state even when calls are concentrated to any specified group by calling a calling object instrument from the other group when the call of a certain group is congested.

CONSTITUTION: When performing detour before transmitting a calling signal 22 from #1 to #n, an alternative routing instruction display signal 25 and an alternative route continuation display signal 26 are transmitted. When the congested state of a certain group is detected, any group which traffic is less than a detour reception allowable value is searched, that group is decided as the alternative routing destination, and an idle machine body number group to be temporarily used for the alternative routing destination group is searched from an alternative routing destination machine body number group managing table. Concerning the configuration of alternative routing destination machine body numbers, a digit larger than '1000' is allocated to a detour destination machine body number group 25c, and digits from '0' to '100' use the values of alternative route source machine body numbers and successively allocated from calling signals #1 to #n. In this case, the calling request to be detoured is generated after the decision of congestion.