PURPOSE: To enable the inner surface of a sample chamber to be sufficiently cleaned by introducing a highly pure gas until a pressure at which glow discharge can be produced within the sample chamber is achieved and producing glow discharge by means of a discharge electrode placed in the sample chamber.

CONSTITUTION: A supply source 8 of a gas for producing glow discharge such as N2 gas or a rare gas (such as helium, argon, neon or xenon), is connected to a sample chamber 2 by means of a conduit pipe 9. A discharge electrode 10 is fixed in such a position which permits it to be moved to a position which does not obstruct the sample chamber 2 during the time when discharge cleaning is not carried out. The pressure of the gas and discharge voltage applied to the discharge electrode 10 are selected so that the dark part of the glow discharge produced is around 0.5mm to several mm and that the entire internal space of the sample chamber is practically filled with plasmas produced by the discharge. In addition, for the purpose of maintaining the purity of the gas during the discharge, introduction and exhaustion of highly pure gas is continuously performed. As an alternate method, after the sample chamber 2 is filled with a highly pure gas, the contaminant gas is selectively adsorbed and exhausted by means of a strong getter material.