A cooling system of a battery which efficiently cools a high voltage battery which is mounted in an electric vehicle or a hybrid vehicle so as to maintain battery performance by using a refrigeration cycle of an air-conditioning system, which is provided with an electric compressor, outside heat exchanger, inside heat exchanger, and a control device. A refrigerant path of the air-conditioning system for running refrigerant is provided with a branch path having a heat exchanger which bypasses the inside heat exchanger, a medium path connected to the heat exchanger runs another refrigerant for cooling the battery, control valves are provided which adjust the amounts of the refrigerant which flows to the refrigerant path and the branch path, and, when the control valves run the refrigerant to both the refrigerant path and the branch path, the control device increases the speed of the electric compressor.