To prevent an erroneous alarm caused by a rapid atmospheric pressure change when a typhoon approaches or the like.

The pressure inside an air chamber 11, which has increased by a temperature elevation caused by a fire and atmospheric pressure reduction caused by the approach of a typhoon or the like, is detected by a pressure sensor 21 and outputted from a pressure change detecting part 25 as a pressure change signal. On the other hand, peripheral atmospheric pressure is detected by a pressure sensor 22 and an atmospheric pressure change signal is outputted from an atmospheric pressure change detecting part 29. The pressure change signal is corrected by the atmopspheric pressure change signal in a fire judging part 26 and a temperature change signal is extracted. When the temperature change signal exceeds a fixed value continuously for a fixed time, the occurrence of a fire is judging and an alarm signal OUT is outputted.