PURPOSE: To control heat flow for a constant temperature container by providing a box having a field applying unit, a superconducting material coupled thermally with a temperature control member and a constant temperature container and applying a field higher than a critical field to superconducting material.

CONSTITUTION: A container 4 of superconducting material and a cover 4a thereof have low thermal conductivity while a constant temperature container 5 and a cover 5a thereof are kept at a constant temperature different from that of a temperature control member 3. When a field higher than a critical field HC is applied to the container 4 and the cover 4a thereof by supplying a current to a coil 2, the superconducting material makes a quick transition from superconducting state to normal state. Since thermal conductivity of the container 4 and the cover 4a increases abruptly under that state, the container 5 and the cover 5a are kept at same temperature as the temperature control member 3. Consequently, temperature of the case 5 and the space 6 can be controlled easily by regulating the temperature of the member 3.