A processing method includes: disposing a workpiece in a processing container of a processing apparatus, and maintaining an inside of the processing container in a vacuum state; providing a cluster nozzle in the processing container; supplying a cluster generating gas to the cluster nozzle and adiabatically expanding the cluster generating gas in the cluster nozzle, thereby generating gas clusters; generating plasma in the cluster nozzle to ionize the gas clusters and injecting the ionized gas clusters onto the workpiece; supplying a reactive gas to the cluster nozzle and exposing the reactive gas to the plasma such that the reactive gas becomes monomer ions or radicals; and supplying the monomer ions or radicals to the processing container, thereby exerting a chemical reaction on a substance present on a surface of the workpiece.