An inorganic/organic composite material which is advantageous in physical characteristics (mechanical strength), chemical characteristics (thermal stability) and electric characteristics (electric conductivity), as compared with a sole system, as being compounded in nano order. An improved energy storage device using the inorganic/organic composite nano-beads, and especially an improved electrochemical capacitor for covering the range of a power energy density which cannot be achieved with devices of the prior art. A method of producing the inorganic/-organic composite nano-beads.The inorganic/organic composite nano-beads have a three-dimensional structure in which nuclei are made of an inorganic material having a particle diameter of nano order and a high electronic conductivity, and are covered with thin films of an organic conductive material.