The substrate processing apparatus is provided with a gas-liquid mixing nozzle for generating a process liquid mist by mixing a liquid and a pressurized gas, to discharge the process liquid mist to a substrate at high speeds. The liquid may be remover liquid, intermediate rinse liquid or deionized water. The reaction products which having been generated on the substrate in etching process is removed at high speeds with the flow of the mist, whereby the quality of the process is improved.