A method of forming a silicon oxide film on silicon exposed on a surface of a workpiece includes mounting the workpiece on a mounting table in a processing chamber; generating plasma of a process gas containing oxygen by supplying the process gas into the processing chamber; applying a bias to the workpiece by supplying high-frequency power to the mounting table; and forming the silicon oxide film by applying the plasma to the biased workpiece and oxidizing the silicon. A ratio of oxygen in the process gas is set to be in the range of 0.1% to 10%. A pressure in the processing chamber is set to be in the range of 1.3 Pa to 266.6 Pa upon forming the silicon oxide film. An output of the high-frequency power is set to be in the range of 0.14 W/cm2 to 2.13 W/cm2 per unit area of the workpiece.