Abstract: The present invention relates to a heat storage element having a latent heat storage material sealed in the container under a reduced pressure. This heat storage element permits effective reduction of air in the heat storage element, and therefore, heat is smoothly transferred from outside to the latent heat storage material through the wall of the container. Accordingly, a heat storage element which has a very high heat storage characteristic is realized. Furthermore, this heat storage element, having almost no air sealed in, has a large heat storage density. When this heat storage element is used in combination with an electric heater element, heat will be uniformly transferred from the electric heater element to the latent heat storage material, involving a very small risk of part of the electric heater element being overheated.