To provide a molten salt battery device and a control method for the molten salt battery device, by which a molten salt battery can be easily heated up, stand-by time can be shortened and energy required to heat up the molten salt battery can be reduced.

A molten salt battery device 1 comprises a plurality of molten salt battery units 3 and an auxiliary battery (power source) 41 operable at room temperature. Each molten salt battery unit 3 includes a heater. Upon startup of the molten salt battery device 1, the auxiliary battery 41 supplies power to the heater of one molten salt battery unit 3, causing the one molten salt battery unit 3 to be heated up and to become operable. The one molten slat battery unit 3 that has become operable supplies power to heaters of other molten salt battery units 3, causing the other molten salt battery units 3 to be heated up and to become operable. By this configuration, the molten salt batteries are easily heated up without requiring a large amount of energy, shortening start-up time of the molten salt battery device 1.