A plurality of forecast weather groups in a period comprising a plurality of days including a forecast target day for forecasting the electric power demand, and a plurality of actual weather groups in a period in a plurality of days in the past are set as the target period, and the similarity between the forecast weather group and the plurality of actual weather groups is calculated, a trend of a subsequent electric power demand is predicted based on the comparison of the plurality of calculated similarities whereby the electric power demand of a forecast target day is known.