A method of reducing resist residue on a semiconductor substrate includes introducing ozone gas into a chemical solution effective to dissolve resist residue adhered to the semiconductor substrate. The chemical solution is circulated through a processing tank where the semiconductor substrate is immersed in the chemical solution and through a first circulation path having a first pump and a first filter. After dissolution of the resist in the processing tank, the chemical solution is circulated through a second circulation path having a second pump and a second filter and returned to the processing tank. The first filter is cleaned by circulating the chemical solution through a third circulation path that includes the first pump and the first filter while introducing ozone gas into the chemical solution. The third circulation path is a closed loop path that excludes the processing tank, the second pump and the second filter.