Abstract: The present invention relates to an apparatus for preventing evaporation of liquefied gas in a liquefied gas reservoir used for cooling an energy dispersive spectrometer type X-ray detector (EDS detector) and its control method. A cold head of a cryogenic refrigerator is disposed in an upper opening of the liquefied gas reservoir, the cryogenic refrigerator is adapted to be put into automatic operation depending on temperature inside the liquefied gas reservoir, wherein the automatic operation mode of the cryogenic refrigerator is put into rest by remote control during the use of the EDS detector, while the cryogenic refrigerator is put into automatic operation with a previous alarm issued when the temperature inside the liquefied gas reservoir increases over a set temperature with the EDS detector being out of use, thereby allowing vaporized gas to be reliquefied with gas consumption reduced.