PURPOSE: To execute easily fault bracketing by a program control utilizing a control bus of a control device itself, by constituting so that a control device which becomes a holding state when a fault is generated can execute access directly to a fault controlled device.

CONSTITUTION: When a fault detecting part 312 detects a fault, a control device 1 and a control device 2 are set to a holding state and an operating state, respectively, by device switching control parts 12, 22. Also, only a control bus switching part except a control bus switching part 311 executes a switching operation so that a control bus 35 from a bus connecting part 24 is connected to a device to be controlled 33. In this way, only a device to be controlled 31 is connected to a control bus 34 of the device 1 which becomes a holding state, and others are connected to the bus 35 of the device 2 which is in operating state, and they are subjected to each control. A control part 21 executes a control program of a usual operation time, but a control part 11 executes a control program for bracketing a faulty part.