To obtain a control method in which the concentration of pure water in an organic solvent inside a container can be controlled efficiently by finding, on the basis of the relative relationship between the boiling point of the organic solvent and the concentration of the pure water in the organic solvent, the concentration of the pure water in the organic solvent from a measured boiling point and comparing it with a judgment reference value.

When a substrate W is dried inside a treatment tank 20, the greater part of pure water sticking to the surface of the substrate W is dropped on a recovery tray 24 so as to be discharged to the outside after the vapor of an organic solvent S is mixed with a condensed liquid. A part is mixed irreversibly with the organic solvent S inside the treatment tank 20, and the concentration of the pure water is increased. Then, it is confirmed that the organic solvent S in a prescribed amount is stored inside the treatment tank 20. After that, the temperature (the boiling point) of the organic solvent S is measured by a thermometer 27. The relative relationship, which is found in advance, between the boiling point of the organic solvent S and the concentration of the pure water is compared by a pure-water concentration derivation means, and the concentration of the pure water in the organic solvent S inside the treatment tank 20 is found. Then, the found concentration of the pure water is compared with a judgment reference value. When the concentration is at an allowable concentration or higher, the organic solvent S is replaced, and the concentration of the pure water is controlled properly.