A structure of a device for automatically controlling a sequence of water treatment to reduce nitrate nitrite ions in water being treated by an electrochemical reaction, to substitute nitrogen gas for the produced ammonia, and thereby to remove the ammonia from the water and a sensing method using such a device for automatically detecting the progress of reduction or denitrification or degradation of the treating capability of the cathode or anode are disclosed. The water treating device comprises a cathode 15 for reducing nitrate nitrite ions by an electrochemical reaction, an anode 16, an electrolytic bath 10 containing the cathode and anode, and a hydrogen gas sensor 30 for measuring the concentration of the hydrogen gas in the electrolytic bath 10. Water to be treated is introduced into the electrolytic bath 10. Current is applied to the water. The end of the reduction of the nitrate nitrite ions or degradation of reduction capability of the cathode 15 can be sensed on the basis of the value measured by the hydrogen gas sensor 30 or the controlled current value of the electrolytic bath 10. |