A chiller control apparatus for controlling a plurality of chillers and supplying cold or heat, the chiller control apparatus comprising: a chiller selection unit configured to select the chiller to be operated from among the plurality of chillers according to a required load; and an operation command unit configured to output an operation signal corresponding to the required load, to the chiller selected by the chiller selection unit, wherein the chiller selection unit comprises: a number-of-chillers change determination unit (112,211,131) configured to determine based on the required load whether to change the number of currently selected chillers; and a chiller determination unit configured to determine a chiller whose operation state is to be changed, based on a relationship between a load of each chiller and an efficiency index value acquired in advance and on the required load when the number of chillers is changed based on a result of determination by the number-of-chillers change determination unit characterized in that the chiller determination unit comprises: an insufficient load value calculation unit configured to obtain an insufficient load value for each currently operating chiller by subtracting the required load from an optimal-load-range high value sum that is a sum of optimal-load-range high values, each of the optimal-load-range high values being set for each chiller as a threshold value of a load at a high load side that is an allowed efficiency range, in a case that the number-of-chillers change determination unit determines to increase the number of chillers; and a chiller-to-be-operated selection unit configured to obtain, for each of chillers that are currently stopped, an efficiency index value when the chiller is operated with the insufficient load value, based on the insufficient load value obtained by the insufficient load value calculation unit and the relationship between the load and the efficiency index value, and select a chiller indicating the highest efficiency index value as a chiller to be operated. a chiller selection unit configured to select the chiller to be operated from among the plurality of chillers according to a required load; and an operation command unit configured to output an operation signal corresponding to the required load, to the chiller selected by the chiller selection unit, wherein the chiller selection unit comprises: a number-of-chillers change determination unit (112,211,131) configured to determine based on the required load whether to change the number of currently selected chillers; and a chiller determination unit configured to determine a chiller whose operation state is to be changed, based on a relationship between a load of each chiller and an efficiency index value acquired in advance and on the required load when the number of chillers is changed based on a result of determination by the number-of-chillers change determination unit characterized in that the chiller determination unit comprises: an insufficient load value calculation unit configured to obtain an insufficient load value for each currently operating chiller by subtracting the required load from an optimal-load-range high value sum that is a sum of optimal-load-range high values, each of the optimal-load-range high values being set for each chiller as a threshold value of a load at a high load side that is an allowed efficiency range, in a case that the number-of-chillers change determination unit determines to increase the number of chillers; and a chiller-to-be-operated selection unit configured to obtain, for each of chillers that are currently stopped, an efficiency index value when the chiller is operated with the insufficient load value, based on the insufficient load value obtained by the insufficient load value calculation unit and the relationship between the load and the efficiency index value, and select a chiller indicating the highest efficiency index value as a chiller to be operated. a number-of-chillers change determination unit (112,211,131) configured to determine based on the required load whether to change the number of currently selected chillers; and a chiller determination unit configured to determine a chiller whose operation state is to be changed, based on a relationship between a load of each chiller and an efficiency index value acquired in advance and on the required load when the number of chillers is changed based on a result of determination by the number-of-chillers change determination unit an insufficient load value calculation unit configured to obtain an insufficient load value for each currently operating chiller by subtracting the required load from an optimal-load-range high value sum that is a sum of optimal-load-range high values, each of the optimal-load-range high values being set for each chiller as a threshold value of a load at a high load side that is an allowed efficiency range, in a case that the number-of-chillers change determination unit determines to increase the number of chillers; and a chiller-to-be-operated selection unit configured to obtain, for each of chillers that are currently stopped, an efficiency index value when the chiller is operated with the insufficient load value, based on the insufficient load value obtained by the insufficient load value calculation unit and the relationship between the load and the efficiency index value, and select a chiller indicating the highest efficiency index value as a chiller to be operated.