Provided is a pure water production method for producing pure water by decarboxylating to-be-treated water under acidic conditions and then deionizing the result by using a reverse osmosis membrane separation device, the pH of inflow water flowing into the reverse osmosis membrane separation device and the water quality of permeated water of the reverse osmosis membrane separation device being measured, and the pH of the inflow water being adjusted on the basis of the measured pH and water quality so that the water quality of the permeated water is within a prescribed range, wherein an operation condition adjusting step is performed for changing the pH of the inflow water by an amount corresponding to a prescribed range, and for adjusting the pH of the inflow water by comparing the average value (average value before the water quality change) of the water quality of the permeated water at a prescribed time after the pH change and the average value (average value after the water quality change) of the water quality of the permeated water during a prescribed period after the point at which the prescribed time has elapsed after the pH change.