PURPOSE: To optimally perform serially different etching by providing a gas diffusion hole shielding plate movably to gas diffusion plates thus allowing a plurality of gas diffusion holes to be partially blocked.

CONSTITUTION: A chamber 10 has an inlet 20 and an exit 22 respectively of etching gas and gas diffusion plates 24 are arranged near an inlet 20 and an anode 12. Introduced etching gas goes through gas diffusion holes 25 to flow under the gas diffusion plates 24 inside the chamber 10 for coming in contact with the surface of a work piece 18 under treatment so as to cause reaction in order to perform etching of the surface of the workpiece 18 under treatment. Further, a gas diffusion hole shielding plate 28 having a smaller diameter than that of the gas diffusion plate 24 is made to be up and down drivev above the gas diffusion plates 24 toward the gas diffusion plates 24 through a supporting arm and a lever by a motor. Depending on the position of the gas diffusion shielding plate 28 being separated from, or in close contact with the gas diffusion plates 24, all of the gas diffusion holes 26 come to a state of being used or part of the gas diffusion holes 20 come to a state of being blocked.