To provide a technology for electric power trade mediation to be used in an electric power trading market for trading in electric power with a power supplier and a demander participating.

The method comprises a step to register the information on the electricity buying area of the demander and information on the predicted quantity of electric power to be used in a memory device, a step to determine the power supply form and power transmission pattern satisfying the predicted quantity of electric power to be used by each demander regardless of the power buying area specified by the demander, and satisfying conditions including the condition for the power transmission cost, and a step to determine the amount to be paid to the power supplier of each power buying area based on the determined power supply form and determining the amount claimed based on the power transmission cost of the electric power between the power buying area and the residence area of the demander and the quantity of used power. The method allows each demander to specify the power buying area to buy the electric power, allows the power supplier of each area to sell the electric power generated by the supplier in the electric power trading market, and allows an mediator to conduct the business.