A corona preionization electrode unit for use in gas laser apparatus, wherein the electric field for corona discharge is concentrated, and ultraviolet radiation is not blocked, and further the laser gas stream is not obstructed, thereby allowing efficient, stable and uniform corona preionization. The corona preionization electrode unit is disposed in a gas laser apparatus together with a pair of main discharge electrodes for ionizing and exciting a laser gas. The corona preionization electrode unit includes a first electrode covered with a dielectric material and a second electrode placed in contact with the outer surface of the dielectric material around the first electrode. The corona preionization electrode unit is positioned in the vicinity of either one of the main discharge electrodes. The second electrode is a plate-shaped member having a straight edge contacting at least the outer surface of the dielectric material. A portion of the plate-shaped member other than the edge is provided with a plurality of openings for passage of ultraviolet light for preionization and a circulating laser gas.