A gas-producing apparatus that produces hydrogen or oxygen, includes: an electrolysis cell that produces hydrogen and oxygen through electrolysis of water; and a gas-liquid separation device that is connected to the electrolysis cell and that receives the water and the hydrogen or the oxygen, wherein the gas-liquid separation device includes a first chamber and a second chamber, the first chamber and the second chamber are separated by a partition wall, and a lower part of the first chamber and a lower part of the second chamber communicate with one another.