A gas detector (10) includes a cell internal space (130) into which a target gas is supplied, the target gas exhibiting an absorption peak in an absorption spectrum; a light source (410) configured to generate light having at least a wavelength belonging to the absorption peak; and a photodetector (420) configured to detect the light that has emitted from the light source (410) and has propagated through the cell internal space (130). The gas detector (10) further includes a conductive thin film (220) in which a plurality of optical apertures (222) are regularly arranged such that a transmission peak in a transmission spectrum is superimposed over the absorption peak in the absorption spectrum along a wavelength axis. The conductive thin film (220) is provided on an optical path extending from the light source (410) to the photodetector (420), and is provided so as to be contactable with the target gas within the cell internal space (130).