PURPOSE: To make etching operation by stabilized negative ion emission performable, by setting up more than two sheets of drawer electrodes in position between an ion generator and a sample, while biasing a first electrode at the ion generator side into being negative and a second electrode at the sample side into being positive, respectively.

CONSTITUTION: A rodlike cathode 22 is placed on the central part of a cylindrical anode 21 and at the lower part, a first electrode 23, a second electrode 24 and a third electrode 25 are all set up after being insulated from the anode 21. And, with a bias power source 28, the first electrode 23 is biased into being negative, while with a bias power source 29, the second electrode 24 is biased into being positive. As for the third electrode 25, it is grounded. With this constitution, a large areal negative ion suitable for ionic etching can be stably secured.