PURPOSE: To form a surface to be etched into a mirror surface state without emulsification by selectively etching a silicon substrate in an anode coupling type dry etching apparatus using mixture gas of SF6 gas and O2 gas of a range having specific mixture ratio of O2 gas.

CONSTITUTION: Gas of mixture of sulfur hexafluoride as reaction gas with oxygen gas in a range of 6 to 15vol.% to the entire reaction gas is used. In a reaction tank 1, a lower electrode 2 as a stage and an upper electrode 3 are opposed. A discharge tube 4 provided dispersively at the peripheral edge of the bottom plate 11 of the tank 1 is provided toward the center of the electrode 2 parallel to its bottom plate. A silicon substrate 20 to be processed, formed with a mask pattern is mounted on the stage 2 of a dry etching apparatus, SF6 gas and O2 gas are fed from an inlet 7 as mixed reaction gas 8 while regulating its flow rate. A high frequency power is applied to the electrode 3 to generate a plasma 9 between the electrodes 2 and 3.