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Leak test by sniffing method is widely used and the leak rate is traceable to the SI unit via a leak detector and a leak artefact. Recently, a gas mixture, for example a gas mixture of 5% hydrogen and 95% nitrogen, is used as a tracer gas. In case of the leak artefact, the gas flow rate is reduced by the gas restrictor, such as cramped capillary. At the narrow path, depended on the conditions, the gas flow mode is in molecular flow and the composition of the gas mixture may be changed due to the difference of the velocity of molecules. In this study, the stability of the composition of the gas mixture was investigated by measuring and comparing the conductance of the leak artefact. As a result, the stability was confirmed at the generation of the steady gas flows within 50 runs.