To provide underwater nitrogen removal equipment which is simple and efficient without requiring large-scale equipment for biological treatment, ion-exchange and the like in order to remove nitrogen in water.

At least a pair of insoluble electrodes 3 are inserted into nitrogen-containing water in a treatment water tank 1 and are energized to remove nitrogen in water by electrolysis or electrolysis and evacuation. By removing nitrogen in water by electrolysis, nitrogen removal in industrial wastewater, well water and the like is also made possible.