PURPOSE: To ensure the dynamic change of priority when an external device monitors each device by adding a priority mask register having bit correspondence to each device.

CONSTITUTION: The priority mask registers (MSK0WMSKn+1) 12 decide the processing priorities to the processing mechanisms 11. The monitor and processing are carried out first to the device corresponding to the bit where '1', for example, is set among those registers 12. Then the contents of the registers 12 are changed according to the result of said execution and the patrol priority is dynamically changed. Thus the monitor processing is accelerated to the fault of a main body device. At the same time the response is accelerated against the fault and the reliability of a patrol priority control system is improved.