To control the temperature of a molten salt bath more stably than the conventional case, to improve the service life of a heat exchanger by suppressing its breakage due to heat cycle fatigue, and to realize excellent thermal efficiency as a whole in heating electrolytic cells.

A plurality of electrolytic cells each provided with a heat exchanger immersed into a molten salt bath are arranged in parallel. Each heat exchanger is provided with: a cooling fluid feeding apparatus; an air intake-side duct continuing from the cooling fluid feeding apparatus; a plurality of conductors continuing from the air intake-side duct; and an exhaust-side duct continuing from the plurality of conductors and continuing to the cooling fluid feeding apparatus. A burner unit for heating is provided in the inside of the air intake-side duct, and an air introduction part for cooling is formed between the inner wall of the air intake-side duct and the burner unit for heating. A blower motor constituting the cooling fluid feeding apparatus is inverter-controlled.

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