PURPOSE:To obtain a fire detector capable of recognizing distinctly a fire in a monitoring area by deciding the detection of the fire by calculating the ratio of the respective emission energies obtained through two filters with different characteristics. CONSTITUTION:A sector 2 is provided with a lambda1 filter 21A, which is hard to be influenced by CO2 in the air near a room temperature and transmits only the measured wavelength of the emission band of CO2 in a flame, and a lambda2 filter 22A which transmits only a reference wavelength band, not influenced by CO2, etc., in the fire. As revolving a motor 6, the respective emission energies of the measured wavelength 4.5mum and the reference wavelength 3.8mum are fetched into a detecting element 3 alternately from the filters 21A and 22A, and amplified by a pre-amplifier 4, and inputted to an operation part 5. The operation part 5 calculates the ratio of both the energies by a deciding means 51 based on a chopper synchronizing signal, obtained in a position detector 7, and decides the detection of the fire. Because the ratio at the time when the flame is detected is different from the ratio at the time when the flame is not detected as shown in a figure, the distinct detection of the fire in the monitoring area can be realized.