PURPOSE: To maintain high performance stably for a long period by outputting an overheating state detection signal when an output signal of a band pass filter for passing only sound of a special sound satisfies predetermined conditions.

CONSTITUTION: Sound from a sound generator is received by a microphone 20 of a sound receiver 3. In this case, peripheral noise is included in the sound received by the microphone 20. However, a decision detector 18 of the receiver 3 passes only a special frequency through band pass filters 22a-22c and judges that the sound generator for detecting the overheated state is operated only when the state that sound pressure levels of all the special frequencies exceed the set value is continued for a predetermined time. The receiver 3 outputs a signal indicating the overheated state in which a position to be monitored out of the receiver 3 is a predetermined temperature or higher by closing a contact 29. Thus, high performance can stably be maintained for a long period.