To obtain high reliability by detecting a fire with the utilization of both an infrared ray of a wavelength specific to the fire and an infrared ray of a near wavelength, and make an apparatus inexpensive by setting two infrared sensors.

The detector consists of a wavelength sensor A1 of an infrared wavelength of 4.4 μm specific to a fire, a wavelength sensor B2 of an infrared wavelength near the wavelength of 3.9 μm, a judgment circuit 4 and the like. The detector judges a fire with the utilization of both sensors. For instance, when an incandescent lamp is turned on with several Hz, an input frequency of the infrared ray satisfies a fire condition, but an energy amount of the inputted infrared ray holds that A (CO2 resonance radiation band) is smaller than B, whereby the detector does not judge it as the fire. When a heating soldering iron is vibrated with several Hz, although the input frequency and energy amount of the infrared ray both satisfy the fire condition, a difference of A and B energy amounts is not larger than a set value, whereby the detector does not judge it as the fire. On the other hand, in the case of a fire, since the input frequency and energy amount satisfy the fire condition and at the same time, the difference is not smaller than the set value, the detector judges it as the fire. In other words, the fire is detected from whether or not the difference of energy amounts of infrared rays of two sets of the sensors A1 and B2 exceeds the set value.