Abstract: An apparatus for diluting a solution comprises a preparation vessel for mixing a solution with a diluent to dilute the solution, a solution supply unit, a diluent supply unit, and a controller. The controller controls the solution supply unit and the diluent supply units to supply the solution and diluent into the preparation vessel in such quantity that the concentration of the diluted solution is higher than a desired concentration. The controller also calculates an additional quantity of the diluent necessary for diluting the solution in the preparation vessel to the desired concentration based on a difference between the concentration of the solution in the preparation vessel and the desired concentration, and controls the diluent supply unit to add the diluent into the preparation vessel in smaller quantity than the calculated additional quantity and repeats the calculation and addition until the solution concentration in the preparation vessel reaches the desired concentration.