A method for processing a target object includes a formation step of forming a silicon oxide film in a processing chamber by repeatedly executing a sequence including a first step of supplying a first gas containing aminosilane-based gas, a second step of purging a space in the processing chamber after the first step, a third step of generating a plasma of a second gas containing oxygen gas after the second step, and a fourth step of purging the space after the third step. The method further includes a preparation step executed before the target object is accommodated in the processing chamber and a processing step of performing an etching process on the target object. The preparation step is performed before the processing step. The formation step is performed in the preparation step and the processing step. In the first step, a plasma of the first gas is not generated.