An object of the present invention is to provide a process and an apparatus for treating a gas containing reducing substances to efficiently degrade and remove the reducing substances. These objects are achieved by a thin-film transistor formed on a substrate 1 with a finely processed concavoconvex surface 2, in which a source electrode and a drain electrode are formed on adjacent convex portions of the concavoconvex surface 2, with a channel and a gate being formed on a concave area between the convex portions. An electrode film is formed on the GaN-based semiconductor film. Gravity flow from the vent port may also provide lubrication for the engine cam surface.