21 formed on the semiconductor substrate 20 by thermally oxidizing the semiconductor substrate 20 and having a thickness of about 40 nm a silicon nitride film 22 formed on the silicon dioxide film 21 having a thickness of about 120 nm and a photoresist 23 formed on the silicon nitride film 22 and patterned by means of photolithography the object 1 is dry etched by means of the dry etching apparatus illustrated in fig2 etching conditions are determined in the following ranges first as illustrated in fig4 b the silicon nitride film 22 is dry etched while the silicon nitride film 22 is being etched the etching controller or end point detector 13 monitors how much degree the dry etching proceeds based on the signals transmitted from the optical sensor 12 monitoring a light having a wavelength of 388 nm as mentioned later when an intensity of the light suddenly varies the etching controller 13 judges it is time for the dry etching to finish and stops the dry etching apparatus to operate for instance by closing the gas supplier 11 and stopping the high frequency power source 8 by virtue of the etching controller 13 it is possible to stop the dry etching when the etching reaches an interface between the silicon nitride film 22 and the silicon dioxide film 21 as illustrated in fig4 b then the patterned photoresist 23 is removed followed by thermal annealing thereafter the silicon nitride film 22 and the silicon dioxide film 21 are removed thus there are.