PURPOSE: To simplify the structure of a cooling device in order to make the cooling unit compact by applying two independent cooling units, liquid and evaporation cooling units, and by cooling cold plates directly in case of evaporation cooling.

CONSTITUTION: In case of liquid cooling, a circulating refrigerant 'a' is sent to cold plates 1A by a circulating pump 2, and heat-exchange takes place while the refrigerant passes through the cold plates to cool electronic devices 6. The warmed circulating refrigerant 'a' is cooled by heat-exchanging with ram air 'b' in an exterior plate heat exchanger 3, and then returned to the cold plates 1A. When the performance of the exterior plate heat exchanger 3 drops and the temperature of the electronic apparatus 6 rises over a set value, the circulating pump 2 stops and the cold plates 1A are directly cooled by the latent heat of evaporation of the refrigerant in a heat exchanger 4 for evaporation cooling. The evaporated refrigerant is discharged from a relief valve 5. To adopt the relief valve 5 allows free adjustment of the boiling point of the refrigerant for evaporation cooling by changing the pressure of the refrigerant.