An electric power demand adjusting system restrains fluctuation in an incentive for adjusting the demand for electric power or fluctuation in an electric power demand adjustment amount caused by an incentive. The system includes a central apparatus, lower units, and computing machine systems of consumers, which are connected through a communication network. The computing machine system of the central apparatus has a device for predicting a demand, a device for determining the electric power demand adjustment amounts to be sent to the lower units on the basis of the predicted demand and information that includes historical data regarding the electric power demand adjustments of the lower units, and a device for transmitting the determined electric power demand adjustment amounts to the lower units. The arrangement restrains the fluctuation in incentives required for adjusting the demand for electric power.