When LNG leaks from a storage tank, the LNG vapourizes vigorously above the adiabatic concrete floor inside the safety dike and the cryogenic methane diffuses in the atmosphere. It is well known that as the density of a vapourized gas drops, the atmospheric diffusion is enhanced due to buoyancy, and the concentration of vapourized gas along the ground decreases. The present paper is concerned with the ability of Hi-Ex (High Expansion Foam) to raise the temperature (decrease the density) of vapourized cryogenic gas, since the Hi-Ex system is usually applied as a fire protection system at LNG facilities, In the experiments, the liquid nitrogen pool was used to simulate LNG leakage, because the boiling point of liquid nitrogen is-close to that for LNG. The small difference in the heat capacity of vapourized gas was taken into account in the evaluation of experimental results. The temperature variations with time of the vapourized gas which penetrated through the Hi-Ex layer were measured. Furthermore, the solidification of the Hi-Ex layer and the formation of flow passages in the Hi-Ex layer, and the change of evaporation rate of liquid after the dispersion of Hi-Ex onto it were examined in detail. Based on these experimental results, the heat balance among Hi-Ex, liquid, and vapourized gas was discussed, and the amount of Hi-Ex required to keep raising the temperature of vapourized cryogenic gas to that at which the density was almost the same as that of atmosphere was estimated.