PURPOSE: To provide comfortable hot heat environment by a method wherein a container is filled with an inorganic hydrate latent heat accumulating material having overcooling phenomenon and is contained in a floor togetherwith a heater, radiation of heat during a daytime is suppressed and radiation of heat during night is increased.

CONSTITUTION: Sodium hydrogenphosphate dodecahydrate 1 having overcooling phenomenon is contained as a latent heat accumulating material in a container 2. The container 2 is mounted under a floor finish material 4, and a heater 3 and a heat insulating material 5 are provided therebelow. When a heater 3 is energized by using a midnight point, the heat accumulating material 1 is heated. Since, after completion of energization, the latent heat accumulating material 1 indicates overcooling phenomenon, radiation of heat is suppressed in a state to accumulate heat. Thereby, radiation of heat from the heat accumulating material 1 is reduced during a daytime and the increase of a room temperature is suppressed. When temperature is reduced with the decrease in a room temperature in the evening, the overcooling state of the heat accumulating material 1 is collapsed. Fused liquid is re-crystallized and radiation amount, simultaneously, is increased.