Abstract: A leak rate measuring device contains a strip spectrometer in which the ion path of the respective gas is influenced by at least one variable influencing quantity. When a gas having a predetermined mass is detected, and leakages of a gas having other predetermined masses interfere with this detection due to lack of selectivity of the spectrometer, the influencing quantity is modulated in a sinusoidal manner, and the wanted signal is subsequently selected in a lock-in amplifier. This modulation enables, for example, the elimination of the interfering influence of underground water during the leak rate measurement while using helium as a test gas.