PURPOSE: To discharge excessive liquid nitrogen outside a vessel, and enable the reserved amount thereof to be maintained at an optimum level by providing the bottom of an insulated vessel with a liquid nitrogen discharge pipe having a valve mechanism to open and close on buoyancy via the liquid nitrogen.

CONSTITUTION: During cooling operation, liquid nitrogen fed from a spray nozzle 11 is gasified, and cools the inside of an inner cylinder 4a and active carbon adsorbing floor 5. A part of the liquid, however, drops to the bottom of an insulated vessel 4 and is reserved therein. According to the progress of a cooling process, the liquid level of the reserved liquid nitrogen rises, and gives buoyancy to a float 18. As a result, the float 18 rises along a guide 19 and forms a gap with a valve seat 16, thereby discharging the liquid nitrogen to an evaporator 20 via a liquid nitrogen discharge pipe 17. In this case, gas is discharged from an outlet nozzle 22 continuous to a nitrogen gas outlet pipe 15, and the internal pressure of the evaporator 20 becomes equivalent to that of the insulated vessel 4, thereby assisting an inflow of the liquid nitrogen. Then, when a liquid level in the vessel 4 drops, the float 18 closely adheres to the valve seat 16, thereby interrupting the discharge of the liquid nitrogen. The liquid nitrogen discharged to the evaporator 20 is gasified, due to heating by a heater 23 and discharged to an exhaust system D via the nozzle 22.