An object of the present invention is to provide a process and an apparatus for treating an aqueous medium at low running costs with a decreased electricity required for hydrothermal electrolytic reaction. The present invention proposes a two-step process comprising a first step of performing a conventional hydrothermal reaction or a conventional electrolytic reaction prior to hydrothermal electrolytic reaction to degrade readily degradable substances or ammonia or various electrolytically degradable organics and then a second step of performing hydrothermal electrolytic reaction to degrade the remaining organics, whereby the electricity consumed by hydrothermal electrolytic reaction in the second step can be greatly reduced.