An ozonizer has a flat plate-shaped low voltage electrode 7, a flat plate-shaped high voltage electrode 3 facing a main surface of the low voltage electrode 7. The ozonizer also has a flat plate-shaped dielectric 5 and a spacer for forming a discharge gap 6 of a thin thickness in a laminating direction provided between the low voltage electrode 7 and the high voltage electrode 3, a high voltage electrode cooling unit 2 for forming a cooling water passage 2c insulated from the high voltage electrode 3 inside the high voltage electrode 3. An alternating voltage is applied between the low voltage electrode 7 and the high voltage electrode 3 and a discharge is produced in the discharge gap 6 injected with oxygen gas to produce ozone gas. <IMAGE>
