A substrate processing method that processes a substrate including a processing target layer, an intermediate layer, and a mask layer as stacked in that order. The intermediate layer includes an Si-ARC (Si-containing Anti-Reflection Coating) film and the mask layer has an opening exposing a part of the Si-ARC. The substrate processing method includes a shrink etching step during which an opening width reduction process and an etching process are performed concurrently. In the opening width reduction process, deposits are formed on a sidewall surface of the opening of the mask layer by a plasma generated from a gaseous mixture of an anisotropic etching gas and one of a depositive gas and H2 gas. And in the etching process, the Si-ARC film forming a bottom portion of the opening are etched.