The output of a CW type CO₂ laser beam is modulated into square waves and then irradiated to a metal plate. Thermo elastic wave appeared at the irradiated part and propagated into the metal plate. A PZT is mounted on the back side of the metal plate to detect thermo elastic wave. The phase angle of thermo elastic wave changes with the modulation frequency. In this paper, the modulation frequency ƒ₀ has been obtained at which the phase angle becomes 0 degree. When the thickness of the metal plate is l and the thermal diffusivity λ the relation of λ = 0.769ƒ₀l ² is obtained. If l is known, λ can be obtained by measuring ƒ₀. This method is believed allow as to measure λ of not only pure metals, but also alloys.