To provide a gas analyzer capable of precisely measuring the temperature of a sample.

The gas analyzer comprises a measurement chamber 1 having a mounting stand 1a that a substrate W with a sample m absorbed is mounted, a vacuum pump for reducing a pressure in the measurement chamber 1, a halogen lamp 4 for heating the substrate W with the sample m absorbed, a mass spectrometer 8 inserted in the measurement chamber 1 for detecting gaseous molecules of the sample that are desorbed by the increase of a temperature; and temperature measurement systems 11, 16 that measure the temperature of the substrate W using an interferometer 11 that detects the optical thickness of the substrate W. The temperature of the sample m is equal to that of the substrate W. The temperature of the sample m can precisely be obtained by measuring the temperature of the substrate W using the interferometer 11.