An odor concentration is measured by energizing an odor sensor connected across a power supply through a load resistance. A total resistance of the odor sensor is represented by and includes a parallel connection of a first resistive component having a value according to surrounding environmental conditions and a second resistive component having a value according to an odor concentration, and a saturation resistive component connected in series with the parallel connection of the first and second resistive components. A CPU is used to measure the odor concentration by obtaining the value of the second resistive component from an output of the odor sensor which is a voltage across the load resistance. Also, the measured odor concentration can be normalized and compared with a given threshold for fire detection.