In an improved ozonizer, at least those parts of an ozone gas delivery path located downstream of an ozone generating cell which are to come into contact with ozone gas are either composed of or coated with at least one ozone-resistant, Cr-free material selected from among aluminum (Al), an aluminum alloy, Teflon, fluorinated nickel, a nickel alloy, a silicon oxide based glass and a high-purity aluminium oxide. The ozonizer is capable of producing ozone that is not contaminated with Cr compounds at all or which is insufficiently contaminated to cause any practical problem in the fabrication of highly integrated semiconductor devices.