This paper describes a method of obtaining the thermal diffusivity of a metal sample of known thickness. By irradiating an intensity-modulated laser beam of 8 W to the sample at power density of 1.5 × 10⁸ W/m², thermoelastic waves thus produced are detected with a PZT film attached to the back face of the sample. The thermal diffusivity can be obtained by using modulation frequency characteristics of phase contrast of the detected thermoelastic waves. A relation f₀ =1.3 λl^-2 is confirmed experimentally, where f₀ denotes modulation frequency, l metal thickness and λ thermal diffusivity. This relation coincides well with the calculated result obtained on the basis of the theory developed by Jackson and Amer. We hope a measuring apparatus to the thermal diffusivity of a metal plate with thickness of 1.0 × 10^-4 m ∼ 2.0 × 10^-4 m becomes available for a practical use because of its simple structure as well as easy operation.