A heat storage device (1) is constructed by connecting a first container (4) and a second container (5) by a spring (6). The first container (4) has an inner wall of the shape corresponding to a cylinder bore wall (7). A soft structure section (8) is provided at the inner wall. The soft structure section (8) is formed of an elastic resin. The second container (5) has the same structure as the first container. When a heat storage medium in the first container (4) and the second container (5) absorbs heat and changes its phase from solid to liquid, the volume of the heat storage medium increases. The increased portion of the medium forces out the soft structure section (8) to separate the first container (4), the second container (5), and the cylinder bore wall (7) from each other.