Abstract: Disclosed is a magnesium metal-air battery in which capacity of a negative electrode made of magnesium or its alloy is sufficiently utilized for battery performance and which has a positive electrode material which is capable of coping with the capacity of the negative electrode. The magnesium metal-air battery includes at least one unit battery cell. The cell comprises a negative electrode made of magnesium or its alloy; a positive electrode-side catalyst layer including, as positive active material, activated carbon for absorbing oxygen in air, anhydrous poly-carboxylate, manganese and metal powder; a positive current collector which is made of conductive material and which is laminated on the positive electrode-side catalyst layer; and a separator which allows passing of ions between the negative electrode and the positive electrode-side catalyst layer while it separates therebetween. The positive electrode-side catalyst layer may further include carbon black, metal chloride and graphite.