The objective of the present invention is to suppress overshooting of the exit temperature of a heat transfer medium. A device for controlling a heat source device (10a) is provided with a computation unit (22) and a set temperature alteration unit (23). The computation unit (22) calculates a load change rate using a predetermined computation formula. The set temperature alteration unit (23) determines whether or not the load change rate calculated by a unit for calculating the amount of fluctuation (22) exceeds a predetermined threshold, and when same is exceeded, alters the set temperature for cold water exit in the direction of suppressing changes in the cold water exit temperature. For example, when the cold water exit temperature is decreasing, by setting the set temperature for cold water exit to a higher value, the load of the heat source device decreases and the cold water exit temperature is controlled in a manner so as to match the post-alteration set temperature for cold water exit. As a result, it is possible to suppress overshooting.