The present invention provides a method of manufacturing a semiconductor device. The method includes stacking a SiO2 film, a N-containing stopper film, and a resist pattern in this order on a semiconductor substrate, performing etching on the stopper film and the SiO2 film with a F-containing etching gas, with the resist pattern serving as a mask to form an opening, and performing ashing on the resist pattern to remove the resist pattern, using a gas containing an oxygen gas and an inert gas under the condition that the ratio of the oxygen radical to the inert-gas radical becomes equal to or lower than 5.