It is an object to provide a method for manufacturing a semiconductor device that has a semiconductor element including a film in which mixing impurities is suppressed. It is another object to provide a method for manufacturing a semiconductor device with high yield. In a method for manufacturing a semiconductor device in which an insulating film is formed in contact with a semiconductor layer provided over a substrate having an insulating surface with use of a plasma CVD apparatus, after an inner wall of a reaction chamber of the plasma CVD apparatus is coated with a film that does not include an impurity to the insulating film, a substrate is introduced in the reaction chamber, and the insulating film is deposited over the substrate. As a result, an insulating film in which the amount of impurities is reduced can be formed.