A centrifugal-chiller comprising: a variable-speed centrifugal-compressor configured such that its rotational frequency can be varied by an inverter and which is arranged to compress a refrigerant; a constant-speed centrifugal-compressor connected in parallel with the variable-speed centrifugal-compressor and arranged to be operated at a constant rotational speed to compress the refrigerant; a condenser that is arranged to condense the refrigerant compressed by the variable-speed centrifugal-compressor and/or the constant-speed centrifugal-compressor into a liquid using cooling water supplied thereto; an expansion valve that is arranged to expand the refrigerant that is condensed into a liquid by the condenser; an evaporator that is arranged to evaporate the refrigerant expanded by the expansion valve; and a control unit that is configured to control the operations of the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor, wherein, in the case where a required load factor of the centrifugal-chiller is less than a predetermined value, the control unit selects a variable-speed-compressor priority operating mode in which only the variable-speed centrifugal-compressor, whose coefficient of performance in the region where the load factor is less than the predetermined value is higher than the coefficient of performance of the constant-speed centrifugal-compressor, is activated, and in the case where the required load factor of the centrifugal-chiller is the predetermined value or greater, the control unit selects a combined-use operating mode in which the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor are activated. a variable-speed centrifugal-compressor configured such that its rotational frequency can be varied by an inverter and which is arranged to compress a refrigerant; a constant-speed centrifugal-compressor connected in parallel with the variable-speed centrifugal-compressor and arranged to be operated at a constant rotational speed to compress the refrigerant; a condenser that is arranged to condense the refrigerant compressed by the variable-speed centrifugal-compressor and/or the constant-speed centrifugal-compressor into a liquid using cooling water supplied thereto; an expansion valve that is arranged to expand the refrigerant that is condensed into a liquid by the condenser; an evaporator that is arranged to evaporate the refrigerant expanded by the expansion valve; and a control unit that is configured to control the operations of the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor, wherein, in the case where a required load factor of the centrifugal-chiller is less than a predetermined value, the control unit selects a variable-speed-compressor priority operating mode in which only the variable-speed centrifugal-compressor, whose coefficient of performance in the region where the load factor is less than the predetermined value is higher than the coefficient of performance of the constant-speed centrifugal-compressor, is activated, and in the case where the required load factor of the centrifugal-chiller is the predetermined value or greater, the control unit selects a combined-use operating mode in which the variable-speed centrifugal-compressor and the constant-speed centrifugal-compressor are activated.