A method of manufacturing electric double layer capacitors is disclosed. The method assumes a model in which solute is dissolved in solvent before preparing electrolyte, and estimates a withstanding voltage through a simulation. The electrolyte, of which withstanding voltage is expected to exceed a target value, is selectively prepared. The method adjusts respective surface areas of the positive electrode and the negative electrode of the capacitor for making full use of the withstanding voltage of the electrolyte. According to this method, a time for developing electrolyte can be substantially shortened, and an electric double layer capacitor having a high withstanding voltage can be efficiently developed.