Thin magnesium plate 101, which contains metal magnesium, is enclosed by separator 102, which is made of fluid-permeable material and is used as magnesium fuel assembly 100 in magnesium battery 120 in this invention. Magnesium fuel assembly 100 is enclosed from both sides by cathode 103 and provided with electrolyte retention unit 106, which stores electrolyte 107, at its bottom. When magnesium fuel assembly 100 is pushed down from above, separator 102 is impregnated with electrolyte 107, thereby initiating the battery reaction.