The leak rate measuring device contains a strip spectrometer (8) in which the ion path (21) of the respective gas is influenced by at least one variable influencing quantity. Should a gas of mass M4 be detected, and leakages of the gas of masses M2 or M3 interfere with this detection due to a lack of selectivity of the spectrometer, the invention provides that the influencing quantity is modulated in a sinusoidal manner, and the wanted signal is subsequently selected in a lock-in amplifier. This enables the elimination of the interfering influence of underground water during the leak rate measurement while using helium as a test gas.
