PURPOSE: To improve the reliability by constituting a fault switching system so that a frame pulse is always generated and inputted as a frame synchronizing signal.

CONSTITUTION: Data inputted to an existing office data circuit terminating equipment through a subscriber's line is inputted to a line equalizing part 11 and a correction of an attenuation and a waveform distortion at the time when the existing equipment 1 is switched to a stand-by equipment 2 is executed, an obtained equalization parameter is inputted to a storage circuit 4 and stored, and thereafter, inputted to a stand-by equalizing part 21, and also, the equalization parameter from the equalizing part 11 is inputted to a frame synchronizing circuit 13, the synchronization between input data required for switching to the stand-by equipment 2 from the equipment 1 is established firmly and a frame pulse for executing an alignment of a frame pattern is outputted, the pulse is stored in a generating circuit 5 and a new frame pulse is generated and it is outputted to a stand-by synchronizing circuit 23. In the stand-by equipment 2, a subscriber's line is equalized by using the equalizing part 21 from the circuit 4 and a new frame pulse is inputted to a synchronizing circuit 23 from the circuit 5, and the establishment of the synchronization and the alignment are execute, by which the reliability of the system can be improved.