PURPOSE: To check many operating data by a few comparing circuits, and to raise the reliability of a digital control device by comparing the compressed operating data in plural arithmetic units, and checking its soundness.

CONSTITUTION: A data is inputted to a master arithmetic unit 2a and a checking arithmetic unit 2b of a digital control device from an input device 1, and input data 3a, 3b are brought to operation processings 4a, 4b, respectively. In this unit 2a, the necessary one of a processing data 5a is outputted to an output device 6. When checking the soundness by this control device, plural necessary data are selected from the input data 3a, 3b and the processing data 5a, 5b by both the units 2a, 2b, and the operation processing required for each data is performed, by which one or the prescribed number of compressed data are generated, and used as data compression processings 8a, 8b. Subsequently, the same compressed data 9a, 9b are generated by the respective internal data, these data 9a, 9b are compared by a comparator 7, and when an error for showing a significant difference is detected between both the data, the error is outputted.