A gas is introduced into a vacuum chamber after the vacuum chamber is evacuated, and a plasma is generated within at least part of the vacuum chamber. The specimen surface is exposed to the plasma so that the surface is treated. A plurality of different gases, such as SF.sub.6, N.sub.2, and the like, are used as the gas being introduced. The quantity of the gas is changed during the surface treatment. A controller is used as a mechanism for changing the quantity of gas introduced. The controller is operated in accordance with a predetermined program, or by signals obtained by detecting the surface conditions of the specimen during the surface treatment.