Abstract: In a metallic Mg electrolyzing device provided with a plurality of electrolytic cells for forming metallic Mg, promotion of current efficiency in each electrolytic cell and decrease in electric power consumption rate are aimed at. In order to transport the Cl.sub.2 gas produced as a byproduct in the plurality of electrolytic cells to a Cl.sub.2 gas refining equipment, gas from the plurality of electrolytic cells is collectively sucked by use of a blower installed in the main pipe. And an automatic valve is installed in each of the branch pipes which branch out from the main pipe and introduce the Cl.sub.2 gas from each electrolytic cell to the main pipe. Each automatic valve is gate type, and its positioner is driven/controlled by air pressure. The suction pressure for sucking Cl.sub.2 gas in the plurality of electrolytic cells is detected and the opening degree of the automatic valves is automatically controlled so that each detected pressure is controlled to the target value.