In a slab laser, a gas mixture containing carbon dioxide CO2 is formed as a laser-active medium in a discharge space which is formed between two plate-shaped metal electrodes, the flat faces of which are located opposite one another. A resonator mirror is arranged on each of the mutually opposite end faces of the discharge space, the mirrors forming an unstable resonator parallel to the flat faces. At least one of the mutually facing flat faces is provided either on the entire flat face with a dielectric layer the thickness of which is greater on at least one sub-surface than in the remaining area of the flat face, or the at least one flat face is provided with a dielectric layer exclusively on at least one sub-surface.