An energy storage device includes positive and negative electrodes; positive and negative lead wires connected to the positive and negative electrodes, respectively; a separator composed of unit fibers and positioned between the positive and negative electrodes to electrically insulate the positive and negative electrodes from each other; a housing accommodating the positive and negative electrodes and the separator; an electrolyte received in the housing; and positive and negative terminals connected to the positive and negative lead wires, respectively, wherein an electrolyte permeability index of the separator is larger than an electrolyte permeability index of the electrodes, and the unit fibers of the separator are arranged irregularly so that pores formed in the separator have cross sections of polygonal shapes. Using this energy storage device, the electrolyte of electrodes that gives a direct influence on electric capacity is not depleted. Also, stress failure of the separator may be prevented effectively.