PURPOSE: To allow other controller to perform control between high-priority and low-priority devices if one controller has a fault, by providing a common diagnostic part which has a different power source to the plural controllers provided between the high-priority and low-priority devices.

CONSTITUTION: Between plural top-priority devices 6, 11W and low-priority devices 7, 12W, #1, #2W controllers 1, 2W which control the low-priority devices 7, 12W in parallel by commands and signals from the high-priority devices 6, 11W are provided. Those plural controllers 1, 2W are provided in common with a common diagnostic part 13 which has a power source 14 for diagnosis. While the controllers 1, 2W are powered up or in operation, the diagnostic part 13 diagnoses the controllers 1, 2W upon occasion. When a fault of one controller is found by the diagnosis, other controllers control the high-priority and low- priority devices connected to the faulty controller, and thus while the control system is simplified, the power consumption is reduced.