In a molten salt battery device, molten salt batteries are arranged in a container to cause a space to be present around the molten salt batteries, and a heating medium is filled into the space around the molten salt batteries. When an electrothermal heater is used to control the temperature of the heating medium through a temperature controlling section, the heating medium is caused to flow. Between the flowing heating medium and the molten salt batteries, heat is exchanged, whereby the molten salt battery device controls the temperature of the molten salt batteries. Since the molten salt batteries attain the heat exchange with the heating medium, which surrounds the batteries, the internal temperature thereof is evenly controlled. Moreover, the molten salt battery device makes it possible to lower the temperature of the heating medium to cool the molten salt batteries easily.