An object of the present invention is to provide a process and an apparatus for treating a gas containing reducible substances to efficiently degrade and remove the reducible substances. As a means to achieve this object, the present invention provides a process for treating a gas containing reducible substances by hydrothermal electrolysis, comprising supplying a gas containing reducible substances into a reactor charged with an aqueous medium containing a halide ion under application of a direct current at a temperature of 100 DEG C or more but the critical temperature of said aqueous medium or less and at a pressure that allows said aqueous medium to be kept in the liquid phase.