A power control method for a power supplying unit for supplying power from a commercial power supply and a battery to a load. The method includes (a) calculating a deviation amount between an actual power demand and a predicted power demand in a predetermined unit period on the basis of power demand transition data and power demand prediction data, (b) correcting, by a computer, a leveling target value on the basis of the calculated deviation amount, and (c) controlling the power supplying unit so that the power supplying unit supplies power corresponding to the corrected leveling target value from the commercial power supply. The power demand transition data is stored in a power database, and the power demand prediction data is stored in a prediction data storing unit. The leveling target value is a target value of power to be supplied from the commercial power supply.