PURPOSE: To constitute the ozone generator in such a manner that ozone can be generated therein and to enable cleaning of the inside of a water tank by the generated ozone.

CONSTITUTION: This ozone generator has a net-like electrode 7 consisting of a polygonal flat plate within a container 5 which has apertures 3, 4 to allow the flow of gas therein at both ends and is coated with a thin-film electrode 6 on the outer peripheral surface. A voltage is impressed to the thin-film electrode and the net-like electrode with the net-like electrode as anode to cause the net-like electrode 7 to generate the ozone by the electric discharge in the oxygen-contg. gas. An air pump is connected to one aperture of the ozone generator.