An ozonizer has a flat plate-shaped low voltage electrode and a flat plate-shaped high voltage electrode facing a main surface of the low voltage electrode. The ozomzer also has a flat plate-shaped dielectric and a spacer forming a discharge gap in a laminating direction, located between the low voltage electrode and the high voltage electrode, a high voltage electrode cooling unit forming a cooling water passage insulated from the high voltage electrode inside the high voltage electrode. An alternating voltage is applied between the low voltage electrode and the high voltage electrode and a discharge is produced in the discharge gap, into which oxygen is injected, to produce ozone gas.