A metal-air battery includes: a cell module configured to generate electricity by oxidation of a metal and reduction of oxygen and water; a water vapor supply unit configured to supply a first water vapor to the cell module; a moisture storage unit configured to supply a first moisture at a first flow rate to the water vapor supply unit; and a condensing unit configured to supply a second moisture at a second flow rate to the water vapor supply unit by condensing the water vapor condensed from the cell module.