A gas analyzing apparatus includes a measurement chamber having a mounting member for mounting thereon a substrate on which a sample is adsorbed; a depressurizing mechanism for depressurizing the inside of the measurement chamber; and a heating unit for heating the substrate having the adsorbed sample thereon and mounted on the mounting member. The apparatus further includes: a mass spectrometer inserted in the measurement chamber, for detecting gas molecules escaping from the sample with an increasing temperature; and a temperature measuring unit for measuring a temperature of the substrate having the adsorbed sample thereon by using an interferometer which detects an optical thickness of the substrate.