A new procedure to estimate thermal diffusivity has been developed by using a solution explicitly obtained from an inverse problem of one-dimensional unsteady heat conduction. This method has merit being independent of surface condition and is simpler than the existing methods, which commonly use a direct heat conduction solution and are strongly subject to a boundary condition. The thermal diffusivity can be estimated by using a change in the temperatures, which includes some errors at two different points as uncertainty in the measurement. The value estimated thereby is found to be in good agreement with that of the tested materials, while thermal conductivity needs a reference to give a known heat to the tested material. This method makes the simultaneous measurement of both thermal diffusivity and thermal conductivity possible by using a reference material.