The object of the present invention is to provide a water treatment device in which temperature does not affect performance in nitrogen removal. Water circulates in a first anaerobic filter bed chamber 5, a second anaerobic filter bed chamber 10, a contact aeration chamber 14, a sedimentation chamber 19, and an electrolytic chamber 59 for phosphorus removal. The supernatant in sedimentation chamber 19 flows into a disinfection chamber 21 and is then sent to an electrolytic chamber 100 for nitrogen removal. An electrode pair is provided inside electrolytic chamber 100. Halogen ions present in the water are oxidized on the anode side. After halogen gas is generated, this gas reacts with water and generates hypohalous acid. On the cathode side, the nitrogen components in the water are converted to nitrate ions, and these nitrate ions are further converted to ammonium ions. The hypohalous acid and ammonium ions react, and the nitrogen components are converted to nitrogen gas by way of chloramine.