To change a purifying function of a decomposition, deodorization and sterilization of a harmful gas to a high efficiency by generating ozone by providing an electrode in a tube of a single cold electrode discharge tube and a cage type electrode at an outside of the tube at its opposite side and by imparting a high frequency voltage between both electrodes.

The discharge tube 4 has an incorporated cold electrode 1 as an anode and has a cage-shaped cylindrical cage type electrode 2 at the outside of the discharge tube 4 at its opposite side. The ozone is generated by imparting the high frequency voltage between both electrodes 1 and 2 from an inverter, by occurring an induction discharge between the cage type electrode 2 and an inside of the discharge tube 4 and by exciting an around oxygen of the cage type electrode 2. The far-ultraviolet ray is generated by a discharge phenomenon between both electrodes 1 and 2 since a far-ultraviolet generating substance is applied at an inside of the discharge tube 4. The harmful gas is flowed from an ozone generating part direction (right side of figure) of the discharge tube 4, irradiated with the discharge tube 4 while being oxidized by being mixed with the ozone and brought into contact with a titanium oxide plate 5 having a photocatalytic activity and purified completely and discharge from the titanium oxide plate 5 while being oxidized again.