The theory and the experimental results on interferometry determination of thermal diffusivity are given. The method is based on interferometry registration of an unsteady temperature field in a flat transparent layer of a medium. Using Clausius-Mossotti equation and the long-time asymptotic behavior of an exact solution to the one-dimensional unsteady heat conduction equation, the formula has been derived which relates the thermal diffusivity of substance to the time derivative of some function of the number of fringes. Measurements for air have been made. The obtained results are compared with the known experimental data.