as the intensity of the secondary ion is increased and saturated to a definite value this phenomenon is utilized thus it becomes possible to decrease the etching velocity allowing only a small amount of the gas of one type to be adsorbed on the specimen surface without decreasing the current density and also to achieve stable and easy control of the constant etching velocity in this instance the gas pressure which causes the change in the etching velocity should be varied with the type of the material to be etched and with the current density of the ion beam and if numerous types of stable oxides exist as is the case with some metals the etching velocity and the intensity of the secondary ion undergo changes in two or three steps referring to fig2 the present invention includes the following structures in addition to those found in the prior art as illustrated by fig1 numeral 19 designates a secondary ion drawing out system 20 a mass separator 21 a detector 22 the secondary ion monitor 23 a gas inlet section 24 and 25 slits and 26 a vacuum exhaust system through the gas inlet section 23 attached to a specimen chamber 13 high purity gas such as oxygen or the like is introduced up to a gas pressure of 1 u00d7 10 5 torr if this introduced gas is allowed to.