To provide a technique by which a sample gas inlet section can be realized for enabling a gas analyzer utilizing membranes to satisfactorily analyze a sample gas, even if the quantity of the gas is very small.

The gas analyzer is provided with a supply space 6A connected to a continuous sample gas supply line 3' via a stop valve 5, a pressure-reduced gas transfer space 30 connected to the space 6A via a first membrane 13, and a measurement space 19 connected to the space 20 via a second membrane 18. The analyzer is also provided with a mass spectrometer 17 for analyzing the gas in the measurement space 19. The supply space 6A is constituted in a cell chamber, that is provided with a septum 14 and can freely produce an airtight state. In addition, a sampling pump 9, which enables the sample gas to be introduced into the cell chamber 6A and a pressure-reducing pump 10 which can freely reduce the pressure in the chamber 6A are installed to the chamber 6A.