A harmful gas treatment apparatus and a water treatment apparatus uses an electrochemical device provided with a solid electrolyte membrane having ion conductivity. A first electrochemical device provided with an anode on one surface of a hydrogen ion conductive electrolyte membrane and a cathode on the other surface thereof is combined with a second electrochemical device provided with an anode on one surface of a hydroxyl ion conductive electrolyte membrane and a cathode on the other surface thereof. Both cathodes are disposed so as to face each other within an electrochemical reaction tank. Each of the cathodes is provided with TiO2 as a metal oxide and Pt as a platinum group supported on a porous body having functions to occlude, concentrate and reduce harmful substances as a reducing catalyst. Thus, a water vapor partial pressure and an oxygen partial pressure on the both cathodes is reduced, making it possible to enhance hydrogen generation efficiency at the normal temperature and constant current.