Provided is a water purifier comprising: a filter unit which has a reverse osmosis membrane filter; a supply valve; an instantaneous heating device which is provided with an inflow port, and an outflow port, and which heats and purified water entering through the inflow port and flowing to the outflow port, so that hot water is discharged through the outflow port; an extraction member; a supply pump; a domestic water drain line; a hot water drain line; a hot water drain valve; and a control unit which controls the opening/closing of respective flow channels of the hot water drain line and the domestic water drain line so that, among a hot water draining process through the hot water drain line, and a domestic water draining process through the domestic water drain line, the hot water draining process is carried out first.