A workpiece is processed uniformly, e.g., a uniform plated film is formed on a workpiece, by a processing liquid, e.g., a plating solution, which is maintained in an optimum state while minimizing the amount of the processing liquid to be used. An apparatus for processing the workpiece has a plurality of solution supply tanks 50a, 50b for individually holding a plurality of solutions, respectively, to be mixed into a processing liquid 54 while managing the temperatures of the solutions, a plurality of mixing tanks 52a, 52b for mixing the solutions individually supplied from the solution supply tanks 50a, 50b into the processing liquid 54 while managing the temperatures of the solutions, and a processing bath 56 for introducing the processing liquid 54 therein and processing the workpiece, e.g., a substrate W, by bringing the workpiece into contact with the processing liquid 54 while managing the temperature of the processing liquid 54.