In the present equipment for spectroscopic measurement of a gas in a gas mixture, for example a gas in the atmosphere, a filter device is used to filter out one spectral line from other spectral lines belonging to other gases and the transmitted light is detected by a detector which gives a signal to devices for calculating and displaying the measurement data. The filter device consists of an interference filter (1), at least one fixed etalon (2) and a Fabry-Perot interferometer (1). In a preferred design, the filter can be fitted with two fixed etalons instead of one.