PURPOSE: To have a retaining force for an electrode plate and its uniform distribution compatibly by forming a boss for pressing the electrode plate to an electrolytic plate from a plurality of protruding pieces supported in the form of cantilever.

CONSTITUTION: In a gas channel 31 provided with a boss for pressing electrode plates 5, 7 to an electrolytic plate 3, the boss is formed from a plurality of protruding pieces 35 supported in the form of cantilever. Each of such protruding pieces 35 is formed by raising aslant from a base board 33 with three sides cut away and by bending the tip in the direction of the base board 33. As this piece 35 as cantilever basically forms a one-end fix beam in terms of mechanism, the amount of deflection for a certain specified load can be set great, and those pieces deflect independently from one another. Therefore, the specified retaining force is conducted uniformly to the electrode plates 5, 7 while each of the pieces 35 deflects in compliance with eventual dispesion of the piece 35 itself and/or the electrode plates 5, 7 in terms of accuracy.