The present invention provides a water treatment method capable of supplying clean water by sterilizing a filtering membrane as well as the inside of a water tank using sterilizing water. The water treatment method of the present invention comprises: a) a step for draining water inside a storage unit which stores water filtered by a filtering unit; b) a step for sterilizing the storage unit by supplying the sterilizing water; and c) a step for draining the sterilizing water in the storage unit to the outside. A drain line for draining the sterilizing water includes a drain pump which detects a load change and determines the finish of a draining process.
