A method of manufacturing a semiconductor device including forming a mask layer on a polycrystalline silicon film formed on a semiconductor substrate via an insulating film; forming a dense pattern and a sparse pattern on the mask layer to form a mask; etching the polycrystalline silicon film with the mask by controlling a temperature of the semiconductor substrate placed in an etching chamber at 50 degrees Celsius or higher, supplying an etching gas composed of a hydrogen bromide containing gas and a fluoromethane based gas into the chamber, and generating plasma in the chamber.