PURPOSE: To keep current efficiency high over a long time by passing direct current inversely to anode and cathode to electrolytically remove impurities sticking to the surface of the electrodes at the time of obtaining magnesium by electrolyzing magnesium chloride.

CONSTITUTION: Magnesium is deposited on the cathode and chlorine is generated on the anode by electrolyzing magnesium chloride molten salt with monopolar or bipolar type electrode, which are respectively recovered by floating up on the surface of a bath. In the electrolytic refining, direct current is passed inversely to the anode and the cathode at regular or optional interval. The current value inversely passed is preferably 0.1-1 times of that at the normal operation. The soiling such as magnesium oxide or iron sticking to the cathode is decomposed and removed. As a result, short circuit or passivation is prevented, high current efficiency is kept and metal magnesium is produced at a small energy consumption.