The improved method to measure the thermal diffusivity of porous materials such as ceramics is proposed in using an ordinary laser flash method. The penetration of the laser energy into the test piece of porous materials is taken account in this method, by using the derived theoretical equation. The thermal diffusivity of such materials and its measurement errors are calculated simultaneously by fitting the experimental data to this derived equation with a personal computer connected to the laser flash device. The usefulness of this method is shown by comparing it with the conventional method.