In a film formation method for a semiconductor process, a target substrate having a target surface with a natural oxide film is loaded into a reaction chamber, while setting the reaction chamber at a load temperature lower than a threshold temperature at which the natural oxide film starts being stabilized. Then, the natural oxide film is removed by etching, while supplying an etching gas containing chlorine without containing fluorine, and setting the reaction chamber at an etching pressure and an etching temperature lower than the threshold temperature. Then, the reaction chamber is purged. Then, a thin film is formed on the target surface by CVD, while supplying a film formation gas, and setting the reaction chamber at a film formation temperature.