The invention relates to a measuring device and to a method for analysing a test gas by means of an infrared absorption spectroscopy. Said measuring device comprises: a measurement chamber (2) containing test gas which is to be analysed, a laser (1) which is arranged in relation to the measurement chamber (2) such that the light emitted by the laser traverses the measurement chamber (2), a detection device (61) which detects the light emitted by the laser (1) and traversing the measurement chamber (2), and an evaluation unit (8) which evaluates the signals generated by the detection device (61) for determining light absorption in the measurement chamber (2). According to the invention, said laser (1) is a laser emitting in a narrow-band, the line width thereof being less than or equal to the width of an infrared absorption line of the test gas which is to be measured, said laser (1) being embodied and adjusted so that the laser frequency is varied periodically within a defined spectral range, the laser frequency and the variation being selected such that at least one infrared absorption line of the test gas which is to be measured lies in the defined spectral range, and the detection device (61) is embodied and adjusted in order to detect, with temporal resolution, the light emitted by the laser (1) and traversing the measurement chamber (2) in such a manner that the light absorption can be determined, with temporal resolution, within the predetermined spectral range, the detection device (61) carries out an individual absorption measurement within a time span of 10-5 s or faster. Said measurement device is also suitable and adjustable for measuring test gas which is in the form of a respiratory gas of a human or animal, said respiratory gas being replaced in the measurement chamber only under the respiration effect of humans or animals, and the respiratory resistance of the measurement device is less than 60 mbar.
