It was found out that when radicals generated by plasma are fed to a treatment chamber via a plurality of holes (111) formed on a partition plate which separates a plasma-forming chamber (108) from the treatment chamber, and the radicals are mixed with a treatment gas which is separately fed to the treatment chamber, the excitation energy of the radicals is suppressed and thereby the substrate surface treatment at high Si-selectivity becomes possible, which makes it possible to conduct the surface treatment of removing native oxide film and organic matter without deteriorating the flatness of the substrate surface. The radicals in the plasma are fed to the treatment chamber via radical-passing holes (111) of a plasma-confinement electrode plate (110) for plasma separation, the treatment gas is fed to the treatment chamber (121) to be mixed with the radicals in the treatment chamber, and then the substrate surface is cleaned by the mixed atmosphere of the radicals and the treatment gas.