A cooling system (10, 100, 200, 300) for an electronic device, comprising: a device room (14A, 14B) in which a plurality of electronic devices are placed; 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 condenser 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 condenser; a heat exchanger which is provided at a place higher than the evaporator and cools and condenses the refrigerant; a refrigerator for cooling the refrigerant which flows in the heat exchanger; and 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 condenser, characterized by 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, 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) after branched from the circulation line 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 condenser 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 from the circulation line. a device room (14A, 14B) in which a plurality of electronic devices are placed; 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 condenser 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 condenser; a heat exchanger which is provided at a place higher than the evaporator and cools and condenses the refrigerant; a refrigerator for cooling the refrigerant which flows in the heat exchanger; and 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 condenser, characterized by 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, 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) after branched from the circulation line 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 condenser 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 from the circulation line.