A method for treating an aqueous medium which comprises a first step treatment wherein an aqueous medium to be treated is subjected to an ordinary hydrothermal reaction or an ordinary electrolysis, to thereby decompose easily decomposable substances, ammonia or organic compounds capable of being decomposed by electrolysis alone, and a second step treatment wherein the resulting aqueous medium is subjected to a hydrothermal electrolysis reaction, to thereby decompose remaining organic substances. The employment of the method results in marked decrease of the electric power consumption in the second step of hydrothermal electrolysis. The method can be used for providing an apparatus and a method for treating an aqueous medium at a reduced running cost through decreasing the electric power consumption required for hydrothermal electrolysis reaction.