PURPOSE: To eliminate the need for the connection of a slave station and a master station with a private line at all times by transferring an alarm caused in an exchange in the slave station to the master station through a public communication network only at the change of an alarm signal.

CONSTITUTION: When an alarm is issued from the exchange 2, an alarm signal is sent to an alarm detection section 11 through plural alarm signal lines 17 provided on each alarm type. Since the alarm detection section 11 has a function to detect the change in an alarm signal, the alarm detection section 11 detects the change in the alarm signal upon the receipt of the alarm signal and sends the state information of an alarm signal line and information representing the change in the state to an alarm information transmission section 12 as alarm detection information. When the alarm cause in the exchange 2 of a slave station 1 is eliminated and the alarm signal is restored, the alarm detection section 11 detects the change in the alarm signal and sends the alarm detection information to the alarm information transmission section 12. Thus, the setting of a private line is not always required and the installation is saved.