A cooling system comprising a first compressor compressing a refrigerant to cool a pre-determined space; a second compressor disposed on an outlet-side of the first compressor connected with the first compressor in series; an outdoor heat exchanger through which the refrigerant compressed in the first or second compressor is heat-exchanged with external air; an expansion device decompressing the refrigerant condensed in the outdoor heat exchanger; a cooling evaporator evaporating the refrigerant decompressed in the expansion device to supply cool air into the pre-determined space, wherein the cooling system further comprises: a discharge tube extending to a suction part of the second compressor to guide the discharge of the refrigerant compressed in the first compressor; an injection tube in which the refrigerant passing through the outdoor heat exchanger is branched to flow; a supercooling expansion device decompressing the refrigerant flowing into the injection tube; a supercooler in which the refrigerant passing through the outdoor heat exchanger and the refrigerant flowing into the injection tube are heat-exchanged with each other; a bypass tube allowing the refrigerant compressed in the first compressor to bypass the second compressor; and a valve device (125, 185) controlling the refrigerant discharged from the first compressor to allow the refrigerant to be selectively introduced into the second compressor, the valve device (125, 185) comprising a first valve device and a second valve device, wherein the second valve device is installed at the bypass tube and is configured to open to allow the refrigerant discharged from the first compressor to bypass the second compressor, wherein the discharge tube comprises a tube coupling part to which the injection tube is connected, characterized in that the bypass tube extends from the tube coupling part to an outlet-side tube of the second compressor, and the first valve device is installed at a spot between the tube coupling part and the suction part of the second compressor and is configured to open to introduce the refrigerant flowing in the injection tube into the second compressor. a discharge tube extending to a suction part of the second compressor to guide the discharge of the refrigerant compressed in the first compressor; an injection tube in which the refrigerant passing through the outdoor heat exchanger is branched to flow; a supercooling expansion device decompressing the refrigerant flowing into the injection tube; a supercooler in which the refrigerant passing through the outdoor heat exchanger and the refrigerant flowing into the injection tube are heat-exchanged with each other; a bypass tube allowing the refrigerant compressed in the first compressor to bypass the second compressor; and a valve device (125, 185) controlling the refrigerant discharged from the first compressor to allow the refrigerant to be selectively introduced into the second compressor, the valve device (125, 185) comprising a first valve device and a second valve device, wherein the second valve device is installed at the bypass tube and is configured to open to allow the refrigerant discharged from the first compressor to bypass the second compressor, wherein the discharge tube comprises a tube coupling part to which the injection tube is connected, characterized in that the bypass tube extends from the tube coupling part to an outlet-side tube of the second compressor, and the first valve device is installed at a spot between the tube coupling part and the suction part of the second compressor and is configured to open to introduce the refrigerant flowing in the injection tube into the second compressor.