A cooling system (10, 100, 200, 300) for electronic devices, a plurality of which being placed in a device room (14A, 14B), said cooling system (10, 100, 200, 300) comprising: an evaporator which is provided close to each of the electronic devices, and cools exhaust air from the electronic device by vaporizing a refrigerant with heat generating from the electronic device; a cooling tower which is provided at a place higher than the evaporator, cools the refrigerant by outside air, and condenses the vaporized refrigerant; a circulation line in which the refrigerant naturally circulates between the evaporator and the cooling tower. a heat exchanger which cools the refrigerant; a refrigerator for cooling the refrigerant which flows in the heat exchanger; a parallel line (58, 60, 64) which is a flow path for the refrigerant, connected to the circulation line, and is provided so that the heat exchanger has parallel relation with respect to the cooling tower; characterized in that it comprises a parallel control mechanism which controls a refrigerant amount of the refrigerant which is fed to the parallel line (58, 60, 64) from the circulation line; a flow rate regulating device which is provided in a refrigerant gas flow path at a position of the evaporator outlet port of the circulation line, and regulates a refrigerant flow rate; a temperature sensor which detects temperature of air discharged from the evaporator; and a controller which controls the flow rate regulating device wherein the parallel control mechanism comprises an outside air temperature sensor which measures outside temperature, a parallel valve which is provided in the parallel line (58, 60, 64), and regulates an amount of the refrigerant which is refrigerant gas returning from the evaporator and flowing into the heat exchanger, and a parallel control part which calculates capacity of cooling the refrigerant in the cooling tower from a measurement result of the outside air temperature sensor, and regulates an opening degree amount of the parallel valve from a result of the calculation to control a refrigerant amount which is fed to the heat exchanger, wherein the controller controls the flow rate regulating device so that the temperature sensor (50,63,65,67) indicates a predetermined value. an evaporator which is provided close to each of the electronic devices, and cools exhaust air from the electronic device by vaporizing a refrigerant with heat generating from the electronic device; a cooling tower which is provided at a place higher than the evaporator, cools the refrigerant by outside air, and condenses the vaporized refrigerant; a circulation line in which the refrigerant naturally circulates between the evaporator and the cooling tower. a heat exchanger which cools the refrigerant; a refrigerator for cooling the refrigerant which flows in the heat exchanger; a parallel line (58, 60, 64) which is a flow path for the refrigerant, connected to the circulation line, and is provided so that the heat exchanger has parallel relation with respect to the cooling tower; characterized in that it comprises a parallel control mechanism which controls a refrigerant amount of the refrigerant which is fed to the parallel line (58, 60, 64) from the circulation line; a flow rate regulating device which is provided in a refrigerant gas flow path at a position of the evaporator outlet port of the circulation line, and regulates a refrigerant flow rate; a temperature sensor which detects temperature of air discharged from the evaporator; and a controller which controls the flow rate regulating device wherein the parallel control mechanism comprises an outside air temperature sensor which measures outside temperature, a parallel valve which is provided in the parallel line (58, 60, 64), and regulates an amount of the refrigerant which is refrigerant gas returning from the evaporator and flowing into the heat exchanger, and a parallel control part which calculates capacity of cooling the refrigerant in the cooling tower from a measurement result of the outside air temperature sensor, and regulates an opening degree amount of the parallel valve from a result of the calculation to control a refrigerant amount which is fed to the heat exchanger, wherein the controller controls the flow rate regulating device so that the temperature sensor (50,63,65,67) indicates a predetermined value.