We established the SI traceable and inherent thermal diffusivity measurement. The thermal diffusivity is a thermophysical property dependent on temperature and it is obtained as a function of specimen thickness and heat diffusion time determined by the laser flash method. The measurement system in AIST became traceable to length, time and temperature. Thermal diffusivity should not be independent of measurement conditions since it is unique for the material. However, thermal diffusivity measured by the laser flash method sometimes depends on the conditions. To solve this problem, we developed a procedure for the laser flash method. Apparent thermal diffusivity values are first measured by changing pulse heating energy at the same temperature. Then the inherent thermal diffusivity is determined by extrapolating these apparent values at the zero energy pulse heating. It was confirmed that the inherent thermal diffusivity obtained by the procedure is independent of measurement conditions. We verified the effect of the procedure from an international joint research between AIST and Laboratoire national de métrologie et d'essais.