To provide a method and instrument for measuring gas temperature by heterodyne interference method capable of contactlessly measuring variation of a gas temperature with high responsiveness and high precision.

Refractive index change of gas is calculated based on phase change of a beat frequency in a measuring part 20 using heterodyne interference method in which an acoustic optical element 12 for heterodyne is utilized to measure the gas temperature, a Gladstone-Dale constant is found based on a gas composition, and the gas temperature is measured based on a state equation of gas using pressure value of the gas.