PURPOSE: To make processing highly accurate by using mixed gas of sulfur hexafluoride and oxygen with a silicon oxide film provided on one face and a mask provided on the other face when a recess of a predetermined depth is to be formed from a surface of a silicon material by means of dry etching, and by stopping the etching when the generated recess reaches the oxide film.

CONSTITUTION: An Al film 3 is applied to one of surfaces of an Si substrate 1, and the film a on a part 4 where a recess 5 is to be formed is removed by using etching solution such as phosphor nitric acid to expose the substrate 1 at the part 4. Then a parallel flat type dry etching apparatus is used to perform etching so as to remove the substrate 1 on the exposed part and make a recess 5. The etching is done until an SiO2 film 2 which has been provided on the other surface of the substrate 1 is exposed and when the film 2 is visible, the etching is stopped. Thus accuracy at the time of forming a diaphragm for example can be remarkably improved.