A method for measuring a concentration of at least one gas component in a measurement gas, wherein light of a light source is guided along a light path through a measuring volume containing the measuring gas to a detector unit, and the concentration of the gas component is determined from the wavelength-dependent absorption of the light detected there. The light path is guided outside of the measuring volume through a substitute gas held in a closed volume. A substitute gas comprising a substitute gas component in a predetermined concentration is used, and the concentration of the substitute gas component is monitored based on the detected wavelength-dependent absorption, and if the decrease in the concentration exceeds a predetermined degree, then an error message is generated.