1. A demand control system that controls power consumption of a power consuming device when a predicted integration value of power predicted to be consumed by the power consuming device in a demand time period exceeds a power target value being a target value for power to be consumed in the demand time period, the demand control system comprising : an actual data storage unit configured to store actual data in a state where the actual data are classified according to environmental conditions, the actual data being based on a consumption integration value of power actually consumed by the power consuming device; a calculator configured to calculate a predicted integration value predicted to be consumed in a current demand time period, by using the actual data at a start or before the start of the current demand time period, the actual data being based on a current actual environmental condition and extracted from storage contents stored by the actual data storage unit from the actual data storage unit; and a changing unit configured to change the power target value in the current demand time period, based on the predicted integration value calculated by the calculator and on the power target value.