A method for the supply of a constant-concentration ozonated water, employs a unit for producing ozonated water and a system for using the ozonated water at a place separated therefrom, interposing an ozonated water circulating line between the ozonated water producing unit and the place for using the ozonated water, causing the interior of the circulating line to circulate the ozonated water so that the flow rate may be constantly fixed at the outlet of the ozonated water producing unit, monitoring the ozone concentration of the ozonated water in the neighborhood of the outlet, adjusting the amount of an ozone gas supplied to the ozonated water producing unit based on the results of the monitoring, further monitoring the amount of the ozonated water used at the place for using the ozonated water, and controlling the amount of the ozonated water to be produced based on the results of this monitoring.