An infrared-type gas detector is provided with a Fabry-Perot filter through which infrared having a predetermined wavelength is selected to pass, a first detection unit for receiving the infrared having passed through the Fabry-Perot filter and outputting a detection signal corresponding to an absorption amount of the infrared, and a case member having a window portion through which the infrared passes and housing therein the first detection unit and the Fabry-Perot filter in an inner space of the case member. The case member has at least one ventilation portion through which the inner space of the case member is communicated with the exterior of the case member.