PURPOSE: To accurately hold the liquid level of the cooling medium in a measuring Dewar vessel and to prevent the disturbance of the liquid surface thereof in replenishing the cooling medium, by providing a cooling medium replenishing Dewar vessel to connect the same to said measuring Dewar vessel.

CONSTITUTION: A cooling medium replenishing Dewar vessel 23 is provided other than a measuring Dewar vessel 13 for immersing a specimen cell 10. The bottom parts of these Dewar vessels 13, 23 are mutually connected by a communication pipe 16 and the Dewar vessel 23 is constituted so as to be made movable up and down by an elevator mechanism 22. When a sensor 11 detects the lowering in the liquid level of the cooling medium 14 in the Dewar vessel 13, the Dewar vessel 23 rises to perform the replenishment of the cooling medium 14. Because the replenishment of the cooling medium is performed from the lower part of the Dewar vessel at this time, no disturbance of the liquid surface is generated. By this method, the liquid level of the cooling medium in the measuring Dewar vessel is accurately held and the disturbance of the liquid surface in replenishing the cooling medium can be prevented.