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 electrode 3, an electrode cooling sheet 1 provided facing a main surface of the electrode 3 at a side opposite the discharge gap 6 for cooling the electrode 3. The ozonizer also has a thermal conducting/electric insulating sheet 2 sandwiched between the electrode 3 and the electrode cooling sheet 1. An alternating voltage is applied between the low voltage electrode 7 and the electrode 3 and a discharge is produced in the discharge gap 6 injected with oxygen gas to produce ozone gas.