The present invention provides a method for forming a silicon oxide film, with a substantially uniform film thickness and without being so influenced by dense sites and scattered sites in a pattern provided on an object to be processed, while keeping advantageous points of a plasma oxidation process performed under a lower-pressure and lower-oxygen-concentration condition. In this method, plasma of a processing gas is applied to a surface of the object having a concavo-convex pattern, in a processing chamber of a plasma processing apparatus, so as to oxidize silicon on the surface of the object, thereby forming the silicon oxide film. The plasma is generated under the condition that a ratio of oxygen in the processing gas is within a range of 0.1% to 10% and pressure is within a range of 0.133 Pa to 133.3 Pa. This plasma oxidation process is performed, with a plate, having a plurality of through-holes formed therein, being provided between a region for generating the plasma in the processing chamber and the object to be processed.