Disclosed is a method of etching comprising preparing an etching solution containing hydrofluoric acid and nitric acid, and etching while adding nitrite ion or a medium for producing nitrite acid ion to the etching solution. As the medium for producing the nitrite ion, silicon with a high impurity concentration, a mixed acid solution containing hydrofluoric acid and nitric acid having been used for dissolving a great amount of silicon, or gaseous nitrogen dioxide may be used. Preferably, the concentration of nitrite ion in the etching solution is detected based on the concentration of NO.sub.x in the gas phase which is in an equilibrium relation to the nitrite ion in the liquid phase of the etching solution, and necessary nitrite ion are added to the etching solution based on the concentration of NO.sub.x.