A water battery includes a housing and at least one battery cell mounted in the housing, capable of operating by injecting water into the housing at the time of use. The at least one battery cell includes an anode made of a metal material with a lower ionization tendency than that of a magnesium, an anode drawer electrode electrically connected to the anode, a cathode made of a magnesium material, a cathode drawer electrode electrically connected to the cathode, a collector electrode mounted between the anode and the cathode, a sheet member with water absorptivity and water retention, closely attached to the collector electrode, and a fixing member for pressing to each other and fixing together the anode, the collector electrode, the sheet member and the cathode. The sheet member includes an electrolyte containing nitrophenol, sodium, citric acid and polyvinyl alcohol.