Disclosed is an energy storage device having high energy density and excellent power density. For example, electric double layer capacitors, redox capacitors, lithium ion electrolyte type capacitors and devices applying any of them are greatly improved in the energy density without deteriorating their advantages such as high power density, high charge/discharge efficiency and long life. Specifically disclosed is an energy storage device containing a positive electrode, a negative electrode and an electrolyte solution, which device is characterized in that a compound capable of performing a doping/dedoping reaction is present in the electrolyte solution.