A cooling system for cooling a CPU 200 mounted on a printed circuit board 100 within a housing thereof, has a heat-receiving jacket 310, being thermally connected with a surface of the CPU generating heat therein, and for evaporating liquid refrigerant stored in a pressure-reduced inner space with heat generation thereof. A condenser 320 receives refrigerant vapor from the heat-receiving jacket for condensing the refrigerant vapor into a liquid by transferring the heat into an outside of the apparatus. A thermo siphon is used for circulating the refrigerant due to phase change thereof, and the condenser has fine grooves on an inner wall surface thereof along a direction of flow of the refrigerant, and is also formed flat in a cross-section thereof, for cooling the refrigerant vapor from the heat-receiving jacket on the inner wall surface thereof, efficiently.