A sodium-sulfur battery according to the present invention is provided with a reservoir space 100 that retains and solidifies a high-temperature molten material having flowed out of a cell 4, in order to prevent the high-temperature molten material from leaking out of a casing 1, even when an accident occurs to generate the high-temperature molten material inside the casing. The reservoir space 100 can be formed along a perimeter of the casing 1, or alternatively, can be formed inside the casing 1. The reservoir space 100 includes, for example, a composite member 15 of a rigid member 11, a heat-insulating material 12, and a heat-resisting material 13.