An NaS battery module includes a container, a plurality of cells for an NaS battery and sand. The side walls of the container partition the inside and outside of the container in a direction in which the arrangement plane of the cells extends. An upper wall and a lower wall of the container partition the inside and outside of the container in a direction perpendicular to the direction in which the arrangement plane of the cells extends. The lower wall and the side walls are high heat insulating walls whereas the upper wall is a solid low heat insulating wall having a heat insulating property lower than that of the high heat insulating wall. The cells and the sand are housed inside of the container. A variable louver is disposed outside of the upper wall so as to cover the outer surface of the upper wall.