PURPOSE: To detect the faults of hardware and software completely by inputting a fault detecting reference signal to an arithmetic circuit so as to be switched to a normal input and comparing the reference signal itself with a signal obtained through the arithmetic circuit to detect a fault.

CONSTITUTION: Input data 1 and a signal outputted from a reference signal generating circuit 2 for fault detection are inputted to the arithmetic circuit 4 through a switching circuit 3, the output of the circuit 4 is inputted to a comparator 6 through a switching circuit 5 together with the signal outputted from the circuit 2 and the output of the comparator 6 and a self-diagnosis function output 9 outputted from the circuit 4 are inputted to an OR circuit 8. At the diagnosis of a fault, the signal from the circuit 2 is inputted to the circuit 4 through the circuit 3 and the output of the circuit 4 is outputted to the circuit 6 through the circuit 5 to monitor respective circuits. Thus, the abnormality of the hardware and software can be precisely detected and audinary self-diagnosis is also attained on the basis of the output 9.