PURPOSE: To detect abnormality and a fault speedily by converting a digital signal obtained by applying the 1st reference voltag to an AD converter into an analog signal, and detecting whether the signal is between the 2nd and the 3rd reference voltages or not through a comparing circuit.

CONSTITUTION: A timing circuit 3 generates a switch driving signal 14 shown in a figure (b) to supply a reference voltage V2 for checking to the AD converter 1. When the output signal of the AD converter 1 is inputted to a DA converter 2, the driving signal 14 is ceased and a driving signal 15 is generated as shown in a figure (f) to apply a reference voltage V3 to one input of a comparator 4; when it is ceased, a reference voltage V1 is applied again. Namely, the comparator 4 compares the output signal of the converter 2 with the reference voltages V1 and V3, and then V1 again as shown in a figure (g) during and before and after the generation of the driving signal 15. In this case, V1>V2>V3. If the converter 1 breads down and V3> the output signal of the converter 2>V1, the output signal of the comparator 4 is still at "L" or "H" and the breakdown is detected.