A respiration gas monitor (RGM) device includes an infrared light source (30) launching infrared light (34) through a respired air flow path (36), and an optical detector (40) that detects the infrared light after passing through the respired air flow path. An absorption line bandpass filter (42) has a passband encompassing an absorption line of a target gas. A reference line bandpass filter (44) has a passband over which the respired air is transparent. A control device (62, 64, 70, 80, 90) switches the RGM device between: a monitoring state in which the absorption line bandpass filter is in the path of the infrared light； and a calibration state in which the reference line bandpass filter is in the path of the infrared light and the absorption line bandpass filter is not in the path of the infrared light.
