Failure detector for warning device

A fire detector capable of detecting its own malfunction by itself and also capable of quickly announcing a failure of high-level emergency in the fire detector. A plurality of determining values (Vu,Vd) are established for the output level (SLV) of a physical quantity detecting means for detecting the physical quantity of a fire phenomenon such as smoke, and a different time (T1-T4) is set for each of the determining values. A shorter time (T2,T4) is set for greater deviation from the normal value of the output level. It is determined that the physical quantity detecting means is faulty if it is detected that the output level of the physical quantity detecting means continuously exceeds any of the established determining values for not less than the time which has been set for that particular determining value.