Electrorefining of Mg has been investigated in a molten salt system, and the electrolysis conditions for the effective purification have been discussed. A purified mixture of MgCl2-NaCl-CaCl2 at 943K was used as an electrolytic bath. Magnesium metal was dissolved anodically by potentiostatic electrolysis, and purified Mg was electrodeposited at the cathode. The potential during electrolysis was essential for the electrolysis; large anodic overpotential directly caused the deterioration in purity of Mg electrodeposit. It was necessary for the anodic overpotential to be less than 1.0V for good purification, though a certain degree of cathodic overpotential was required for the effective electrodeposition of Mg. In addition to the electrode potentials, some factors affected the electrorefining of Mg metal. Under the suitable electrolysis condition, the Fe content of the Mg electrodeposit was less than 10ppm. The so-called "three-layer" electrolysis has been also tried in this study by using the similar experimental procedure. Magnesium metal in Mg-Cu alloy at the bottom of an electrolytic bath was dissolved anodically, and Mg metal electrodeposited at the cathode simultaneously. It is concluded that an electrorefining technique is effective for the purification of Mg metal.