The respiration gas monitoring (RGM) device includes an infrared light source 30 for emitting infrared light 34 through the breathing air flow path 36 and a photodetector 40 for detecting infrared light after passing through the breathing air channel.The absorption line band pass filter 42 has a pass band including the absorption line of the target gas.The reference line bandpass filter 44 has a passband whose breathing air is permeable.The control devices 62, 64, 70, 80, 90 have an RGM device between the monitored state where the absorption line bandpass filter is in the infrared light path and the calibration line band pass filter in the infrared light path and the absorption line bandpass filter is not within the infrared light path Toggle.呼吸ガス監視（ＲＧＭ）デバイスは、呼吸空気流路３６を通る赤外光３４を放出する赤外線光源３０と、呼吸空気流路を通った後の赤外光を検出する光検出器４０とを含む。吸収線帯域通過フィルタ４２は、対象ガスの吸収線を包含する通過帯域をもつ。基準線帯域通過フィルタ４４は、呼吸空気が透過性である通過帯域をもつ。制御デバイス６２、６４、７０、８０、９０は、吸収線帯域通過フィルタが赤外光の経路内にある監視状態と基準線帯域通過フィルタが赤外光の経路内にあり、かつ、吸収線帯域通過フィルタが赤外光の経路内にない校正状態との間においてＲＧＭデバイスを切り替える。
