PURPOSE: To prevent the accuracy of the title method from becoming bad due to a difference in thermal expansion by a method wherein the temperature of a specimen holder is made to coincide quickly with that of a preliminary chamber (a specimen chamber).

CONSTITUTION: A preliminary chamber 3 whose degree of vacuum is maintained to be equal to that of a specimen chamber is attached to the specimen chamber via a partition door; in addition, a heat-conducting plate 10 whose temperature has been controlled to be nearly equal to that of the preliminary chamber 3 is provided inside the preliminary chamber 3; the heat-conducting plate 10 is brought into contact with a specimen holder 2; and heat is transferred between the specimen holder 2 and the preliminary chamber 3.