A method for forming an oxide film by plasma-assisted processing includes: (i) supplying a precursor reactive to none of oxygen, CxOy, and NxOy (x and y are integers) without a plasma to a reaction space wherein a substrate is placed; (ii) exposing the precursor to a plasma of CxOy and/or NxOy in the reaction space; and (iii) forming an oxide film on the substrate using the precursor and the plasma.