Liquid nitrogen is filled in a low temperature vessel; an ejector that sucks liquid nitrogen by blowing a cooling agent (liquid or gas) such as low temperature helium gas or liquid helium of pressure higher than in the space within the vessel is disposed in the vessel; the liquid nitrogen blown with the cooling agent is cooled by the cooling agent to become fine particles of solid nitrogen which fall down; and gas in a space of the vessel is discharged out of the vessel so as to maintain the pressure of the space higher than the atmospheric pressure. A gaseous phase of liquid nitrogen in an adiabatic vessel is depressurized to vaporize nitrogen in a liquid phase so that the temperature of the nitrogen reaches the triple point of nitrogen by lowering the temperature to thereby produce solid nitrogen by keeping the temperature at the triple point, and that the produced solid nitrogen is transformed into slush by stirring the content of the adiabatic vessel. A super conductive body formed of a material exhibiting a state of super conductance in the vicinity of the temperature of liquid nitrogen or of the temperature where liquid nitrogen and solid nitrogen coexist can be cooled by immersing the body in slush nitrogen held in an adiabatic vessel or flowing in an adiabatic pipe.