There is provided a heat storage system including: an indoor space; a heat storage space which is adjacent to the indoor space and in which a latent heat storage material having a melting point or a freezing point in a range of 5.degree. C. or higher and 30.degree. C. or lower is installed; and a natural ventilator controlling introduction and blocking of outside air to the heat storage space, in which a heat resistance between the heat storage space and the outside air is set to be greater than a heat resistance between the heat storage space and the indoor space.