PURPOSE: To improve the reliability of trouble decision making by smoothing and comparing the outputs of respective processors, deciding whether each processor is in trouble or not on a majority decision basis, and selecting the outputs of processor systems which have no trouble and higher priority according to the result of the trouble decision making and preset priority.

CONSTITUTION: The outputs of the processors 11-13 are inputted to digital filters 21-23 and smoothed, and outputted to a voter 25 at specific timing. The voter 24 compares the smoothed output data and decides whether the outputs of the respective processors 11-13 are abnormal or not by majority decision making. A selector and switch 24 selects the outputs of processors among 11-13 which have no trouble and also have higher priority according to the decision result of the voter 25 and the priority of the specific processors 11-13 and outputs them as a system output. Consequently, natural variance is prevented from being judged as data discrepancy, trouble is accurately judged, and the reliability of the system is improved.