To realizes load frequency control, by predicting the load fluctuation time and the load fluctuation amount of an electric furnace in power demand prediction.

This power demand prediction system is provided with a data input means 124 for inputting electric furnace operation data transmitted via an information transmitting device, an electric furnace load fluctuation prediction means 125 for calculating load fluctuations using the electric furnace operation data, and an output command transmitting means 126 for transmitting demand and supply plan information, based on the load fluctuations to an electric power plant. The electric furnace load fluctuation predicting means 125 classifies changes of the electric furnace operation data into each pattern to store them as actual data, determines the pattern of the electric furnace operation data inputted by the data input means 124, when both of a furnace cover opening/closing signal and an opening/closing signal of an opening/closing means are in closed states, and calculates the power consumption changing amount, after a prescribed time lapse using the actual data of the pattern to prepare a demand and supply plan.