PURPOSE: To make improvement in the accuracy of a sample analysis and the reproducibility of measured values by providing a control means for setting a gate valve isolating a main chamber and a sample chamber from each other at a closed state when the vacuum degree of the main chamber attains a specified value.

CONSTITUTION: Only the main chamber 2 is first evacuated to a vacuum and the air pressure in a vacuum chamber 100 is kept constant until the vacuum degrees of vacuum gages 13, 14 attains the same value. The sample 3 is then transferred from the sample chamber 1 to the inside of the main chamber 2 and only the main chamber 2 is evacuated to a vacuum until the target vacuum degree is attained. The gate valve 4 and all valves are set in the 'close' state so as to prevent the fluctuation in the vacuum degree of the main chamber 2 after the inside of the main chamber 2 attains the target vacuum degree. The analysis and measurement of the sample 3 are then executed. The fluctuation in the vacuum degree of the main chamber 2, if any, is detected by the vacuum gage 14 and is kept monitored at all times and, therefore, the vacuum degree of the main chamber 2 is stabilized at the target value during the time when the sample analysis is executed.