solution obtained is fed to the electrolytic cell 14 as an electrolyte 16 and is electrolyzed to produce chlorine gas c1 hydrogen gas d and caustic soda e the chlorine gas c1 produced is sent to a chlorine drying liquefying unit 18 in which the gas is liquefied to obtain liquefied chlorine f as a product the caustic soda e is purified by a purifying unit 19 to obtain caustic soda g as a product in the zinc chlorine secondary cell 20 zinc chloride contained in its electrolyte 22 is subjected to electrolysis with the d c power supplied from the voltage controller 6 producing zinc as deposited on the negative electrode 21n and chlorine gas c2 at the positive electrode side 21p and lowering the zinc chloride concentration of the electrolyte 22 power in proportion to the amount of deposition of zinc is stored in the secondary cell 20 the chlorine gas c2 produced at the positive electrode side 21p is sent to a chlorine separator 8 in which the gas is separated from the electrolyte the chlorine gas separated off is added to the chlorine gas c1 produced in the common salt electrolytic cell 14 and the combined gas is made into liquefied chlorine gas f by the drying liquefying unit 18 the electrolyte separated from the chlorine gas in the separator 8 is returned to the secondary cell 20 in this way the secondary cell 20 is charged with nighttime power and the chlorine gas c2 generated in.