A first band pass filter selects and transmits light having an infrared wavelength specific to flame. A first infrared-ray receiving element receives the light from the first band pass filter. A second band pass filter selects and transmits light having an infrared wavelength which is not the infrared wavelength specific to flame. A second infrared-ray receiving element receives the light from the second band pass filter. A first output of the first infrared-ray receiving element is compared with a second output of the second infrared-ray receiving element to detect the flame. When light having homogeneous energy in a transmissive wavelength band of the first band pass filter and a transmissive wavelength band of the second band pass filter is incident on the first band pass filter and the second band pass filter, an amount of the light transmitted through the second band pass filter is less than an amount of the light transmitted through the first band pass filter, and the first output and the second output are adjusted so that a level of the first output is equal to a level of the second output.