An electrochemical device comprises, at least, a first electrode, a second electrode, and an electrolyte layer having an ionic conductivity. The first and second electrodes oppose each other by way of the electrolyte layer. The first and second electrodes comprise a composite particle containing an electrode active material, a conductive auxiliary agent having an electronic conductivity, and a binder adapted to bind the electrode active material and conductive auxiliary agent to each other. In the composite particle, the electrode active material and conductive auxiliary agent are electrically coupled to each other without being isolated.