PURPOSE: To automatically execute switching from a main control station to a subcontrol station by constituting the titled system so that a control station sends out a switching signal to the subcontrol station, when a fault is occurred in a main control station, confirms a fact that a confirming signal from the main control station does not exist, and the subcontrol station starts actually a line control.

CONSTITUTION: When a fault is occurred in a main controls station B, first of all, a control station A cannot receive a response signal b1. Therefore, the control station A confirms fact that a response signal b2 from a subcontrol station C can be received, and sends out a switching signal a2' to subcontrol station C. The subcontrol station C receives the switching signal a2', and sends out actually a line control signal c1, only when a confirming isignal from the main control station is not received, and simultaneously, sends out the response signal b2 to the control station A. The subcontrol station C receives in advance line control signals C1, C1', therefore, a reference signal sending-out timing To and line control information can become the same as those of the main control station B. Also, the control station A sends out continuously a switching signal a1' to the main control station B having no response signal.