To early detect a fire without generating any time delay by increasing the detecting sensitivity of CO gas when receiving a hot air flow.

A sensor 10 is configured by opening an opening hole 14 on the surface of a cover 12 which receives a hot air flow, and arranging an electrochemistry type CO sensor 18 in the cover 12 behind the opening hole 14. When receiving a hot air flow containing CO gas generated by a fire, external CO gas is brought into contact with electrolyte solution packed in the CO sensor 18 from the opening hole 12 so as to be detected by an electrode. A detection signal corresponding to CO gas concentration is output from the CO sensor 18, and a fire is discriminated based on the CO gas concentration.