A patterning method comprises a step for forming a first film on a substrate, a step for forming a multilayer film including a resist film on the first film, a step for patterning the resist film by photolithography to form a patterned resist film having a predetermined pattern, a step for forming an silicon oxide film different from the first film on the patterned resist film and the first film by supplying a first gas containing an organic silicon and a second gas containing an activated oxygen species alternately to the substrate, a step for etching the silicon oxide film to form a sidewall spacer on the sidewall of the patterned resist film, a step for removing the patterned resist film, and a step for processing the first film by using the sidewall spacer as a mask.