PURPOSE: To treat the surface of an object to be treated in a short time by obtaining active oxygen of high concentration by using an oxygen/ozone mixed gas containing water when the surface is to be treated by supplying the active oxygen to the surface of the object to be treated.

CONSTITUTION: An oxygen/ozone mixed gas is introduced into a pure water tank 5 through a pipe 3 and is spouted into pure water 15 in the form of a fine air bubble from a ball 18 at a tip of the pipe 3 inserted into the pure water 15. Only the clean oxygen/ozone mixed gas containing a very small amount of pure water 15 as water is introduced into a treatment chamber 1 through the pipe 3. When the oxygen/ozone mixed gas is diffused to the whole surface of a water 8, an excitation reaction of oxygen gas and ozone gas is initiated quickly by means of ultraviolet rays from a low-pressure mercury lamp 11, and active oxygen O (1D) of high concentration is obtained. A resist which has been applied to the surface of the wafer 8 is decomposed to a volatile low molecule such as CO2, H2O or the like. By this setup, the surface of an object to be treated, i.e. the wafer 8, can be treated in a short time.