PURPOSE: To secure the continuation of processing by holding fixed data predetermined in a device to be controlled, and specifying the operation of the controlled system on the basis of the fixed data during stand-by/in-use switching.

CONSTITUTION: The control data of a controller 1 is transmitted to the device 3 to be controlled through a switching circuit 5 and processed by a processing circuit 6, and the device is included in an in-use system. When the switching of the device to be controlled is required owing to a fault, etc., of the device 3, a switching control circuit 4 operates the circuit 5, and disconnects the device 3 from the in-use system to connect a fixed data circuit 7' to a processing circuit 6'. This state is maintained until the circuit 6' receives processing data from the controller 1, and the control operation of a stand-by device 3' to be controlled is determined by the data of the circuit 7' during said period. For this purpose, the contents of the fixed data are selected properly to carry on the operation of the controlled device which is required as a system during the switching operation period although it is limited.