PURPOSE: To reduce running cost by a method wherein two sets simultaneous operation or one set individual operation, which is determined in accordance with a refrigerating load upon starting, is determined by the detection of the outlet temperature of cold water, in a two-set compressor type turbo refrigerating machine.

CONSTITUTION: A number of operating set control circuit upon starting is constituted of output contacts 2, 6, provided in a temperature regulator 17 for detecting the outlet temperature of cold water, and limit switches 9, 10, provided in control motors 19, 20 for controlling a capacity. The output contact 2 is opened at the cold water outlet temperature of 6°C or less and is closed at the same temperature of 15°C or more while the output contact 6 is closed at a temperature higher than 14°C and is opened at the temperature lower than 7°C. The limit switches 9, 10 are closed respectively when opening degrees are lower than the same corresponding to the refrigerating load of 50%. According to this method, a cold water outlet temperature detector for automatic starting and stopping as well as a cold water inlet temperature detector for controlling the number of operating sets can be eliminated compared with a controller so far whereby the simplification of the controller can be contrived.