PURPOSE: To provide a circuit system which is used for performing highly reliable measurement control by preventing the malfunction of the periodically or intermittently operating detector, etc., of an electrical measuring instrument, control equipment, etc., caused by harmful noise.

CONSTITUTION: A malfunction preventing circuit is constituted so that the circuit can automatically discriminate whether or not harmful noise is mixed in signals on the assumption that noise which is superimposed upon signals gets in the signals even when a sufficient protective measure is taken and can stop the adoption of the noise as its signal output when the noise exceeds a certain limit. The circuit is provided with a storage circuit 1 which extracts and holds input signals, comparator circuit 3 which compares the output signal of the storage circuit 1 with input signals during a fixed period of time, detection circuit which detects that the output from the storage circuit 3 exceeds a prescribed range, and periodic sequence circuit 7 which is controlled by the output of the detection circuit.