A water purifier comprising: a filter unit for generating purified water by filtering influent water and discharging concentrated water containing solid substances filtered by the filter unit, said filter unit comprising at least a reverse osmosis membrane filter; a TDS measurement unit provided on a front end of the filter unit to measure TDS of water flowing into the filter unit; a first flow rate measurement unit provided on the front end of the filter unit to measure a flow rate of the water flowing into the filter unit; a second flow rate measurement unit provided on a rear end of the filter unit to measure a flow rate of the purified water discharged from the filter unit; a concentrated water flow rate adjustment valve for adjusting a flow rate of the concentrated water discharged from the filter unit; and a control unit for controlling an opening degree of the concentrated water flow rate adjustment valve on a basis of a measurement value received from the TDS measurement unit, the first flow rate measurement unit, and the second flow rate measurement unit, wherein the control unit adjusts the opening degree of the concentrated water flow rate adjustment valve when a variation amount of the TDS value transmitted from the TDS measurement unit is equal to or greater than a first reference value, and wherein the control unit grasps the flow rate of concentrated water and the flow rate of purified water from the flow rate of raw water and the flow rate of purified water transmitted from the first flow rate measurement unit and the second flow rate measurement unit, respectively, and adjusts the opening degree of the concentrated water flow rate adjustment valve when a variation amount of a ratio of the concentrated water flow rate to the purified water flow rate is equal to or greater than a second reference value. a filter unit for generating purified water by filtering influent water and discharging concentrated water containing solid substances filtered by the filter unit, said filter unit comprising at least a reverse osmosis membrane filter; a TDS measurement unit provided on a front end of the filter unit to measure TDS of water flowing into the filter unit; a first flow rate measurement unit provided on the front end of the filter unit to measure a flow rate of the water flowing into the filter unit; a second flow rate measurement unit provided on a rear end of the filter unit to measure a flow rate of the purified water discharged from the filter unit; a concentrated water flow rate adjustment valve for adjusting a flow rate of the concentrated water discharged from the filter unit; and a control unit for controlling an opening degree of the concentrated water flow rate adjustment valve on a basis of a measurement value received from the TDS measurement unit, the first flow rate measurement unit, and the second flow rate measurement unit, wherein the control unit adjusts the opening degree of the concentrated water flow rate adjustment valve when a variation amount of the TDS value transmitted from the TDS measurement unit is equal to or greater than a first reference value, and wherein the control unit grasps the flow rate of concentrated water and the flow rate of purified water from the flow rate of raw water and the flow rate of purified water transmitted from the first flow rate measurement unit and the second flow rate measurement unit, respectively, and adjusts the opening degree of the concentrated water flow rate adjustment valve when a variation amount of a ratio of the concentrated water flow rate to the purified water flow rate is equal to or greater than a second reference value.