PURPOSE: To maintain the temperature at a constant level inside a refrigerator with a simple constitution by storing coolant in a container. comprising a superconductive object, and covering it with heat insulating material.

CONSTITUTION: The cooling is performed with the heat of vaporization which is taken away as liquid nitrogen 6 vaporizes in a container 1. When the temperature inside the refrigerator 4 becomes lower than the critical temperature TC by being cooled by liquid nitrogen 6, where TC is the critical temperature at which the superconductive object forming the container 1 becomes superconductive, the container 1 is under the superconductive condition, and the temperature of the coolant in the container 1 is hard to be transmitted into the refrigerator 4, which has no effect to further cool a cooled object 5. On the other hand, if the temperature in the refrigerator 4 becomes higher than the critical temperature TC, the superconductive ability of the container 1 is lost, facilitating for the coolant temperature in the container 1 to be transmitted into the refrigerator 4, and the cooling of the cooled object 5 advances quickly. This way, the temperature inside the refrigerator 4 as well as that of the cooled object is always maintained at a constant level near the critical temperature TC.