The present invention provides a water treatment method for minimizing the waste of water and improving sterilization efficiency for sterilizing the inside of a water tank. The water treatment method of the present invention comprises the following: a) a step for allowing sterilizing water generated from a sterilization module to flow into a storage unit; and b) a step for draining the sterilizing water inside the storage unit to the outside. The storage unit comprises a first storage tank and a second storage tank storing water filtered by a filter unit at different temperatures. The a) step is performed by operating the sterilization module while the inflow of water to the inside of the first storage tank or the outflow of water to the outside of the first storage tank is prevented. [Reference numerals] (AA) Water supply
