To position a measuring probe for measuring the particle density in the vicinity of the measuring surface of a measured sample as much as possible, prevent the interference of the hand of an operator with the measuring probe and the interference of the measuring probe with a sample holder of the measuring sample when the sample is replaced.

When a sample replacing cover 13 is closed for measuring the ion density of a measured sample 4, a measuring probe 15 is positioned very near to the ion density measuring position of the measured sample 4 in the vicinity of the sample replacing position within a vacuum container 2. The ion density measured becomes a value very near to the ion density of the measured sample 4. When the sample replacing cover 13 is opened for replacing the sample, the measuring probe 15 evacuates to the outside of the vacuum container 2. The interference of the hand of an operator with the measuring probe 15 and the interference of a sample holder 5a with the measuring probe 15 are prevented in the setting and replacing of the measured sample, and work is made simple and smooth.