To provide a water treatment device that can stably suppress generation of hydrogen sulfide and suppress the generation of scum.SOLUTION: The water treatment device for dissolving an oxygen-containing gas in sewage water from a drainage tank comprises: a drainage device that pumps up the sewage water inside the drainage tank and pumps it to a discharge pipe; and an ejector type gas-liquid dissolving device that is placed in the middle of the discharge pipe. The ejector type gas-liquid dissolving device self-absorbs and mixes the oxygen-containing gas with respect to the sewage water pumped through the discharge pipe to obtain high-concentration oxygen-containing water, and the drainage device has a drainage pump and a drainage pipe connecting the drainage pump and the discharge pipe.SELECTED DRAWING: Figure 1