An electric power supply-and-demand control system includes a memory unit, an estimation unit, a first calculation unit, and a second calculation unit. The system is capable of transmitting/receiving data to/from a power management apparatus provided to each of consumers to instruct a power management apparatus to adjust electric power. The memory unit memorizes a supply, consumptions, reserve power, and response results. The estimation unit estimates a response speed on the basis of the response results. The first calculation unit calculates a target value which allows it to eliminate an unbalance between the supply and a demand. The demand is given as a total of the consumptions. The second calculation unit calculates second adjustment amounts so that a total of the second adjustment amounts approaches the target value and is in the range.