A gas detector, comprising: a space into which a target gas is supplied, the space being defined by a gas cell having a gas inlet and a gas outlet, and the target gas exhibiting an absorption peak in an absorption spectrum; a light source configured to generate light having at least a wavelength belonging to the absorption peak; a photodetector configured to detect the light that has been emitted from the light source and has propagated through the space; a support substrate that is arranged on an optical path extending from the light source to the photodetector; characterized in that a conductive thin film (220, 240, 320) on which surface plasmon polaritons are excited in response to the light from the light source and in which a plurality of optical apertures 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 transmission peak being in accordance with an interval of the regularly arranged optical apertures, the conductive thin film (220, 240, 320) being able to transmit light corresponding to the transmission peak;, wherein the conductive thin film (220, 320) is provided on the support substrate to configure a window member (200, 300) of the gas cell that mediates propagation of the emitted light from the light source to the defined space, or that mediates propagation of the light that has propagated through the defined space to the photodetector the conductive thin film (220, 320) being provided so as to be contactable with the target gas within the defined space. a space into which a target gas is supplied, the space being defined by a gas cell having a gas inlet and a gas outlet, and the target gas exhibiting an absorption peak in an absorption spectrum; a light source configured to generate light having at least a wavelength belonging to the absorption peak; a photodetector configured to detect the light that has been emitted from the light source and has propagated through the space; a support substrate that is arranged on an optical path extending from the light source to the photodetector; characterized in that a conductive thin film (220, 240, 320) on which surface plasmon polaritons are excited in response to the light from the light source and in which a plurality of optical apertures 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 transmission peak being in accordance with an interval of the regularly arranged optical apertures, the conductive thin film (220, 240, 320) being able to transmit light corresponding to the transmission peak;, wherein the conductive thin film (220, 320) is provided on the support substrate to configure a window member (200, 300) of the gas cell that mediates propagation of the emitted light from the light source to the defined space, or that mediates propagation of the light that has propagated through the defined space to the photodetector the conductive thin film (220, 320) being provided so as to be contactable with the target gas within the defined space.