PURPOSE: To allow the contacts of a thermocouple to maintain zero degrees always accurately by detecting the upper and lower limits of the entire volume of the ice and water in a device communicating with a Dewar vessel including the inside of said vessel and controlling the operation of an electronic refrigerating module in accordance with said limits.

CONSTITUTION: When ice i1 grows, the volume V of the ice + water in a Dewar vessel increases and forces the mercury in a U-shaped tube upward to the other end side. A probe B is submerged in the mercury as well. Further when the entire volume of the ice + water attains an upper limit Vu on increasing of the quantity of the ice i, the other end of the mercury arrives at a probe C and microcurrent flows between the probe A and the probe C. This microcurrent is used as a signal and the input circuit 10 of a module 3 is interrupted by said signal, then the ice on a heat sink 2 thaws gradually. The thawing decteases the entire volume V of course until finally the probe B exposes from the other surface of the mercury. The control circuit resumes the original state, the input circuit 10 turne on and the module 3 starts operating again.