PURPOSE: To effectively utilize a spare line by switching a living line to an INS line by a channel bank by originating a call automatically to the preset INS line when a terminal adapter detects the fault of the living line.

CONSTITUTION: Multiplex equipment 1 is connected to plural multiplex equipments 2, 3 by the living lines A, B through the terminal adapters TA. When the terminal adapter TA detects the fault of the living line A, B, it originates the call automatically to the preset INS net 1500, and switches the living line A, B to the line D or the line C connected to the line E by the channel banks of the multiplex equipments 1 to 3. Namely, in the case that communication is executed by the multiplex equipments 1 to 3, the lines A, B are used as the living lines, and the INS net 1500 is used as the spare lines D, E of the lines A, B.