In a gas-phase treating process of a semiconductor wafer using hydrogen, there is provided a technique for safely eliminating the hydrogen in an exhaust gas discharged from a gas-phase treating apparatus. The profile at the end portions of the side walls of gate electrodes of a poly-metal structure is improved by forming the gate electrodes over a semiconductor wafer IA having a gate oxide film and then by supplying the semiconductor wafer 1A with a hydrogen gas containing a low concentration of water, as generated from hydrogen and oxygen by catalytic action, to oxidize the principal face of the semiconductor wafer 1A selectively. After this, the hydrogen in the exhaust gas, as discharged from an oxidizing furnace, is completely converted into water by causing it to react with oxygen by a catalytic method.