In a liquid cooling system, comprising: a heat-receiving jacket for receiving heat from a heat-generation body therein; a heat-radiator for radiating the heat therefrom, which is received within the heat-receiving jacket; and a tank for accumulating refrigerant therein, which is connected with the heat-receiving jacket and the heat-radiator through flow passages, thereby enclosing a refrigerant within the heat-receiving jacket, the heat-radiator and the tank, including the flow passages provided therebetween, and further comprising: a liquid transfer means for generating a circulation flow of the refrigerant enclosed therein, wherein the tank is built up on one side surface of a substrate having superior heat-conductivity and heat-resistance, defined between a flexible film or sheet having superior heat-conductivity and heat-resistance, and the heat-radiator having a heat-radiation flow passage on the other side surface thereof, defined between the flexible film or sheet having superior heat-conductivity and heat-resistance.