A supply system for supplying a functional water to a process unit which treats a substrate using the functional water is provided. In the system, the functional water generated in the functional water generator is supplied to a distributor through a functional water supply pipe. Thereafter, the functional water is supplied to the process unit while the process unit performs a process, and the functional water is returned to the functional water generator through a functional water returning pipe while the process unit does not perform a process. A buffer tank is installed in the functional water supply pipe and the concentration of the functional water is measured in a circulation line connected with the buffer tank. When the measured concentration of the functional water goes out of a set concentration range, the functional water is returned to the functional water generator through functional water returning pipe.