Abstract: A flame detector, in which the generation of a flame is detected by detecting the level of radiant rays at the wavelength characteristic of flame, of infrared rays radiated from the objects generating the flame and in a valley between both of the wavelengths by means of infrared detectors and judging from the comparison of the outputs of the detectors, whether or not there is a valley in the spectrum.The construction of a flame detector according to the present invention can surely detect flame merely by comparing three quantities of radiant rays at three wavelengths without false alarms and accordingly, an element for setting a standard value is not required. This results in a flame detector having a remarkably simple construction.