PURPOSE: To prevent malfunction owing to disturbing radio waves with a simple constitution by providing a protection circuit block separately from a detection circuit and negating the output operation of the detection circuit 5 by the output NOT circuit within the protection circuit block.

CONSTITUTION: When disturbing radio waves above a specified level are received with an antenna 1, the probability that a malfunction is occuring on the detection circuit 5 side owing to these disturbing radio waves is very high. Thereupon, the output of the antenna 1 is high-frequency amplified by an amplifier circuit 2 in a protection circuit block 4 and the output of this amplifier circuit 2 is rectified and smoothed in a smoothing circuit 13. An output OR circuit 3 is operated by the output of this smoothing circuit 13. And the output operation of the detection circuit 5 which is predicted to cause malfunction is negated, thereby suppressing the occurrence of wrong alarming. In this way, there is no need for adding any shield structure to the detection circuit and the malfunction may be suppressed and prevented surely by external disturbing radio waves.