A two-stage refrigeration cycle system comprising: a higher-stage compressor, a higher-stage condenser, a higher-stage expansion device, and a higher-stage evaporator connected in a higher-stage refrigeration cycle by pipes through which refrigerant circulates; a lower-stage compressor, an auxiliary radiator, a lower-stage condenser, a lower-stage expansion device, and a lower-stage evaporator connected in series in a lower-stage refrigeration cycle by pipes through which refrigerant circulates; a cascade condenser including the higher-stage evaporator and the lower-stage condenser and configured to heat exchange between the refrigerant flowing in the higher-stage refrigeration cycle and the refrigerant flowing in the lower-stage refrigeration cycle; and a controller configured to switch between a two-stage operation in which both the higher-stage refrigeration cycle and the lower-stage refrigeration cycle operate and a single-stage operation in which the higher-stage refrigeration cycle stops and the lower-stage refrigeration cycle operates, so as to select one of the two-stage operation and the single-stage operation having a higher COP, wherein CO 2 refrigerant is used in the lower-stage refrigeration cycle, and characterized in that the system further comprises either: an expansion tank is disposed in the lower-stage refrigeration cycle, or a second expansion device is disposed in an upstream portion of a liquid pipe between the lower-stage condenser and the lower-stage expansion device so that the refrigerant passing through the liquid pipe is changed to two-phase gas-liquid refrigerant. a higher-stage compressor, a higher-stage condenser, a higher-stage expansion device, and a higher-stage evaporator connected in a higher-stage refrigeration cycle by pipes through which refrigerant circulates; a lower-stage compressor, an auxiliary radiator, a lower-stage condenser, a lower-stage expansion device, and a lower-stage evaporator connected in series in a lower-stage refrigeration cycle by pipes through which refrigerant circulates; a cascade condenser including the higher-stage evaporator and the lower-stage condenser and configured to heat exchange between the refrigerant flowing in the higher-stage refrigeration cycle and the refrigerant flowing in the lower-stage refrigeration cycle; and a controller configured to switch between a two-stage operation in which both the higher-stage refrigeration cycle and the lower-stage refrigeration cycle operate and a single-stage operation in which the higher-stage refrigeration cycle stops and the lower-stage refrigeration cycle operates, so as to select one of the two-stage operation and the single-stage operation having a higher COP, wherein CO 2 refrigerant is used in the lower-stage refrigeration cycle, and characterized in that the system further comprises either: an expansion tank is disposed in the lower-stage refrigeration cycle, or a second expansion device is disposed in an upstream portion of a liquid pipe between the lower-stage condenser and the lower-stage expansion device so that the refrigerant passing through the liquid pipe is changed to two-phase gas-liquid refrigerant. an expansion tank is disposed in the lower-stage refrigeration cycle, or a second expansion device is disposed in an upstream portion of a liquid pipe between the lower-stage condenser and the lower-stage expansion device so that the refrigerant passing through the liquid pipe is changed to two-phase gas-liquid refrigerant.