The invention provides an improved dry etching method for selectively etching a silicon nitride layer 3 formed on the surface of a SiO.sub.2 layer 2 formed on a p-type semiconductor substrate, the method comprising the steps of supplying a mixed gas of HBr and ClF.sub.3 to a reaction chamber wherein SiBr.sub.4, caused, during the dry etching, by a reaction of the silicon nitride layer 3 and the HBr contained in the mixed gas, partly deposits on an etching wall of the p-type semiconductor substrate 1 while at the same time an excess of the SiBr.sub.4 reacts, between the p-type semiconductor substrate 1 and a wall of the reaction chamber, with the ClF.sub.3 contained in the mixed gas to produce a fluoride. The fluoride thus produced can be easily discharged to the outside, since it is more volatile.