The invention relates to an electrolysis apparatus for producing halogen gases from an aqueous alkali halogen solution, comprising several electrically connected plate-shaped electrolysis cells arranged in a pile and respectively provided with a housing consisting of two half shells made of an electro-conductive material and fitted with outer contact strips on at least one rear wall of said housing, also including two respective planar electrodes (anode and cathode), whereby the anode and cathode are provided with louver-like orifices so that the electrolytic feed material and products can flow though, said anode and cathode are separated from each other by means of a partition wall, are arranged parallel to each other and are electroconductively connected to the associated rear wall of the housing by means of metal reinforcements. The aim of the invention is to provide an apparatus which can operate at flow densities of more than 4 kA/m2 with correspondingly higher production of gas in the boundary layer while maintaining a sustainable service life for the membrane and requiring few pulses. This is achieved by slanting the louver-like orifices (8B, 9B) of the anode (8) and cathode (9) towards the horizontal.