PURPOSE: To reduce a communication tariff by automatically selecting an exchange mode of a more inexpensive communication tariff.

CONSTITUTION: A communication time calculating part 3 calculates a communication time in a line exchange mode based on a data capacitance analyzed by a transfer data capacitance analyzing part 1. On the other hand, a packet number calculating part 4 calculates the number of packets in the using packet length of a packet exchange mode based on the data capacitance analyzed by a transfer data capacitance analyzing part 1. Next, a communication time/distance collating part 5 collates the communication time calculated by the communication time calculating part 3, a distance between a self subscriber's number and an opposite subscriber's number and a first communication tariff data table 7 to extract the communication tariff in the line exchange mode. On the other hand, a packet number/distance collating part 6 collates the packet number calculated by the packet number calculating part 4, the distance between the self subscriber's number and the opposite subscriber's number calculated by a distance calculating part 2 and a second communication data tariff table 8 to extract the communication tariff in the packet exchange mode. An exchange mode deciding part 12 decides either exchange mode of the two, of which communication tariff is more inexpensive than the other.