Abstract: A water purifier comprises: an RO filter including a non-filtration part, a filtration part, and a reverse osmosis membrane for filtering the raw water into the purified water; purified water and raw water storages; a flow channel through which the raw water and the purified water flow; valve modules selectively opened or closed to adjust the flow of the raw water and the purified water; a sterilization module for sterilizing the raw water from the non-filtration part; and a controller controlling the opening/closing of the valve modules and the sterilization module based on flow modes. In a flushing mode for cleaning the reverse osmosis membrane using the purified water, the controller opens/closes the valve modules so that the purified water flows into the non-filtration part and flushing discharge water from the non-filtration part flows toward the raw water storage, and drives the sterilization module to sterilize the flushing discharge water.