An ozone generating system and an ozone generating method producing ozone at a high concentration and operating at high efficiency, in which a raw material gas with no nitrogen added and mainly containing oxygen is used. The amount of generation of NO.sub.X by-product is null. A raw material gas not containing nitrogen and mainly containing oxygen is supplied to an ozone generator, an AC voltage is applied to produce discharge light having a wavelength of 428 nm to 620 nm, a catalytic material containing a photocatalytic material with a band gap energy of 2.0 eV to 2.9 eV is provided on an electrode or a dielectric in a discharge region, gas pressure is kept at 0.1 MPa to 0.4 MPa, and ozone is generated.