A thermal source instrument controlling device includes a setting value calculating portion, a change magnitude calculating portion, and a feed water temperature setting portion that corrects, based on change magnitudes for feed water temperatures calculated by the change magnitude calculating portion, setting values for the feed water temperatures for respective thermal source instruments, calculated by the setting value calculating portion, and sets corrected values in the respective thermal source instruments. The change magnitude calculating portion calculates the change magnitudes for the feed water temperatures for the respective thermal source instruments based on temperature change magnitude information included in instruction data, the setting values for the feed water temperatures set for the respective thermal source instruments, flow rates of the cooling/heating water that flows out from the respective thermal source instruments, and weighting coefficients set for individual thermal source instruments.