A nickel-zinc battery comprising: a positive electrode comprising nickel hydroxide and/or nickel oxyhydroxide; a positive-electrode electrolytic solution comprising an alkali metal hydroxide, the positive electrode being immersed in the positive-electrode electrolytic solution; a negative electrode comprising zinc and/or zinc oxide; a negative-electrode electrolytic solution comprising an alkali metal hydroxide, the negative electrode being immersed in the negative-electrode electrolytic solution; a hermetic container accommodating the positive electrode, the positive-electrode electrolytic solution, the negative electrode, and the negative-electrode electrolytic solution; and a separator exhibiting hydroxide ion conductivity and water impermeability, the separator being disposed in the hermetic container so as to separate a positive-electrode chamber accommodating the positive electrode and the positive-electrode electrolytic solution from a negative-electrode chamber accommodating the negative electrode and the negative-electrode electrolytic solution, wherein the positive-electrode chamber has an extra positive-electrode space having a volume that meets at least part of a variation in amount of water in association with reaction at the positive electrode during charge and discharge of the battery, and the negative-electrode chamber has an extra negative-electrode space having a volume that meets at least part of a variation in amount of water in association with reaction at the negative electrode during charge and discharge of the battery, and wherein the nickel-zinc battery further comprises a gas-liquid flow channel that connects the extra positive-electrode space to the extra negative-electrode space such that the spaces are in gas-liquid communication with each other, and the gas-liquid flow channel allows the positive-electrode electrolytic solution and gas in the positive-electrode chamber to pass through the flow channel into the negative-electrode chamber and also allows the negative-electrode electrolytic solution and gas in the negative-electrode chamber to pass through the flow channel into the positive-electrode chamber in response to a variation in amount of water caused by charge and discharge reactions. a positive electrode comprising nickel hydroxide and/or nickel oxyhydroxide; a positive-electrode electrolytic solution comprising an alkali metal hydroxide, the positive electrode being immersed in the positive-electrode electrolytic solution; a negative electrode comprising zinc and/or zinc oxide; a negative-electrode electrolytic solution comprising an alkali metal hydroxide, the negative electrode being immersed in the negative-electrode electrolytic solution; a hermetic container accommodating the positive electrode, the positive-electrode electrolytic solution, the negative electrode, and the negative-electrode electrolytic solution; and a separator exhibiting hydroxide ion conductivity and water impermeability, the separator being disposed in the hermetic container so as to separate a positive-electrode chamber accommodating the positive electrode and the positive-electrode electrolytic solution from a negative-electrode chamber accommodating the negative electrode and the negative-electrode electrolytic solution, wherein the positive-electrode chamber has an extra positive-electrode space having a volume that meets at least part of a variation in amount of water in association with reaction at the positive electrode during charge and discharge of the battery, and the negative-electrode chamber has an extra negative-electrode space having a volume that meets at least part of a variation in amount of water in association with reaction at the negative electrode during charge and discharge of the battery, and wherein the nickel-zinc battery further comprises a gas-liquid flow channel that connects the extra positive-electrode space to the extra negative-electrode space such that the spaces are in gas-liquid communication with each other, and the gas-liquid flow channel allows the positive-electrode electrolytic solution and gas in the positive-electrode chamber to pass through the flow channel into the negative-electrode chamber and also allows the negative-electrode electrolytic solution and gas in the negative-electrode chamber to pass through the flow channel into the positive-electrode chamber in response to a variation in amount of water caused by charge and discharge reactions.