then as shown in fig5 b the nitride film 3 and the thin gate oxide film 2 in the thick film tr region are etched by a dry etching method the dry etching is executed for example under etching gas ar cf 4 since the dry etching method is used as compared with the conventional method that removes the oxide film by using the chemical solution it is possible to exactly form the boundary between the thin film tr region and the thick film tr region that is since the nitride film 3 is used as the hard mask and the dry etching method is further used it is possible to protect the irregularity in the boundary shape that is caused by the invasion of the chemical solution consequently the improper boundary is not generated whose film thickness is improper such as the end 120 shown in fig2 b and there is no bad influence on the subsequent processes after the etching the resist 4 in the thin film tr region is removed next as shown in fig5 c annealing is executed in the mixed atmosphere of nitrogen and oxygen the annealing condition is for example 900 degrees celsius n 2 o 1 1 and 30 sec since this substrate recovery process is executed through this annealing it is possible to reduce the damage which may be caused by plasma at the time of the dry etching at this time the thin first oxide film 5 a is formed on the surface of the semiconductor substrate 1 in the thick film tr region at the same time the oxide film 5 a u2032 is.