Mass spectrometrical investigation of residual gases in a stainless steel enveloped mass spectrometer, evacuated with a titaniumgetter ion pump, was carried out. Partial pressure of nitrogen of 10-13 mmHg could be measured by means of a secondary electron multiplier which served as an ion detector of the mass spectrometer. Mass 2 (H2) and 28 (N2+CO) were observed to be the dominant constituents of the residual gas. It was also found that considerable amount of argon existed in the residual gas. Hydrocarbon found in the residual gas could be reduced markedly by baking the system up to about 300°C.