The present invention provides a method for forming a silicon oxide film, which has excellent insulating properties and higher quality that can enhance a yield in manufacture of semiconductor devices, while keeping advantageous points in a plasma oxidation process. In this method, plasma is generated under a first process condition that a ratio of oxygen in a processing gas is 1% or less and pressure is within a range of 0.133 to 133 Pa, so as to form the silicon oxide film, by oxidizing silicon on a surface of an object to be processed including silicon as a main component, by using the plasma (first oxidation step). Following the first oxidation step, the plasma is generated under a second process condition that the ratio of oxygen in the processing gas is 20% or more and the pressure is within a range of 400 to 1333 Pa, so as to form an additional silicon oxide film, by further oxidizing the surface of the object to be processed, by using the plasma (second oxidation step).