Since the flow velocity in a free convection from a vertical plate is comparatively small, it may be considered that the influence of the divergent flame propagation on the ignition is negligible except in the case of very low burning velocity, and then the inflammable conditions are determined only by the hot surface. It this experiment, changes in the flow and the composition of mixture around the plate in every experiment under the same conditions are small compared to those in a forced flow. Therefore, it is relatively easy to know the qualitative characteristics of ignition. From the point of view described above, the authors arranged the experimental results making use of the relation between experimental parameters and critical ignition temperature obtained from a numerical analysis of the fundamental equations and showed that the experimental results agreed well with theoretical prediction.