PURPOSE: To decompose an organic substance such as a resist or the like in a short time by obtaining active oxygen of high concentration by using an oxygen/ozone mixed gas containing water and ammonium nitrate when a surface treatment is to be executed by supplying the oxygen to the surface of an object to be treated.

CONSTITUTION: An oxygen/ozone mixed gas is introduced into an ammonia solution tank 5 through a pipe 3a; it is spouted into an ammonia solution 15 in the form of a firne air bubble from a ball 18. Only the clean oxygen/ozone mixed gas which contains a fine particle of ammonium nitrate and water is introduced into a treatment chamber 1 through a pipe 3b. The fine particle of ammonium nitrate in the oxygen/ozone mixed gas comes into contact with the surface of a heated wafer 8 and is pyrolyzed; nitrous oxide gas is generated. A resist which has been applied to the surface of the wafer 8 is oxidized and decomposed quickly to a volatile low molecule such as CO2, H2O, N2 or the like. By this setup, an organic substance such as the resist or the like can be decomposed in a short time.