a first compressor for compressing carbon dioxide as a first coolant;

a radiator for radiating heat from the first coolant;

a first flow control valve for regulating flow volume of the first coolant;

a first evaporator for evaporating the first coolant;

coolant cooling means for cooling the first coolant and including:

a second compressor for compressing a second coolant having an energy consumption efficiency higher than that of the first coolant;

a condenser for radiating heat from the second coolant;

a second flow control valve for regulating flow volume of the second coolant; and

a second evaporator for evaporating, with heat from the first coolant, the second coolant; and

heat-exchange-amount control means for controlling quantity of heat exchanged in the coolant cooling means, wherein

the heat-exchange-amount control means includes:

drying-ratio estimation means for estimating, from a value measured using a sensor, a drying ratio between drying rate of the first coolant exiting the first flow control valve and drying rate when the first coolant exiting the radiator is decompressed to its evaporation temperature,

drying-ratio control-range determination means for determining a control range of the drying ratio, so that a coefficient of performance (COP) value is obtained, in which the difference between the COP value and the maximum COP value obtained when the drying ratio is varied under predetermined operational conditions is within a predetermined ranges, and

control means for controlling the quantity of heat exchanged in the coolant cooling means, so that the drying ratio estimated by the drying-ratio estimation means is within the control range;

the first coolant is circulated through the first compressor, the radiator, the coolant cooling means, the first flow control valve, and the first evaporator, in that sequence; and

the second coolant is circulated through the second compressor, the condenser, the second flow control valve, and the second evaporator, in that sequence.