A processing solution pressurized at a constant pressure is supplied to improve processing accuracy. A controller receives a pressure signal indicating a measured pressure exerted on a processing solution from a pressure sensor, and controls an electropneumatic regulator on the basis of the pressure signal so that pressure on the discharge side of the electropneumatic regulator is constant. The controller decides whether the processing solution contained in a processing solution tank has been depleted on the basis of a difference between a desired pressure and the pressure measured by the pressure sensor, and switches pressurizing shut-off valves accordingly.