An electric power trading support system includes a demand prediction unit, a demand prediction error evaluation unit, a demand predicted value upward adjustment ratio (quantity) setting unit, a private power generation/power purchase ratio (quantities) setting unit, an optimization unit for evaluating optimum values of the demand predicted value upward adjustment ratio (quantity) and the self power generation/power purchase ratio (quantities) based on costs, and a display unit for displaying the optimum values evaluated by the optimization unit, maximizing profit to an electric power retailer by minimizing the cost for power generation and power purchase when the electric power retailer sells electric power by use of a private power generator and backup electric power purchased from an electric power company.