Provided is a multistage method and apparatus for concentrating a solution by reverse osmosis, comprising the steps and means for: maximizing the concentration of absolute in a solution in a multistage apparatus having only standard capacity pumps, including steps of providing first concentrating means for concentrating a solution to a first concentration, said first concentrating means comprising at least one concentrating unit which positioned upstream with respect to a direction in which a solution to be concentrated flows, and providing second concentrating means for concentrating the solution that has been concentrated by first concentrating means to a second concentration which is higher than said first concentrating means comprising at least one concentrating unit which is positioned downstream with respect to said direction; said concentrating units comprising consisting essentially of respective membrane modules and respective standard capacity pumps, the membrane module of the concentrating unit of said first concentrating means comprising a tight reverse osmosis membrane with a high rejection percentage sufficient to achieve a high concentration of said solution with a high flux density of solvent flowing through the membrane, and the membrane module of the concentrating unit of said second concentrating means comprising a loose reverse osmosis membrane with a lower rejection percentage sufficient to easily achieve a further concentration of said first concentrating means with a high flux density of solution flowing through the membrane.