PURPOSE: To perform the exhaust of gas, the separation of electrolyte from gas, and further the reflux of separated electrolyte favorably with simple structure, in the exhaust structure of such type as to let gas flow in horizontal direction.

CONSTITUTION: Exhaust structure is composed of a plurality of exhaust structure elements being provided inside a cover and juxtaposed horizontally in opposition to a cell chamber. The exhaust structure element 30 is equipped with a gas intake chamber 31, a reflux chamber 32, and a passage chamber 33. The gas intake chamber 31 communicates with the reflux chamber 32 through a slit 81, and the reflux chamber 32 communicates with the passage chamber 33 through a slit 82. An obstruction plate 9 lies in the gas intake chamber 32, and pin-shaped bodies 12 and 13 lie at the ceilings 52a and 53a of the reflux chamber 32 and the passage chamber 33, and a reflux hole 10 lie in the bottom plate 42a of the reflux chamber 32, and the reflux hole 10 communicates with a cylindrical body 11. The bottoms 43a and 42a of the reflux chamber 32 and the passage chamber 33 are inclined toward the reflux hole 10.