To acquire a high-output electrochemical energy storage device the range of operating voltage of which is large, a positive electrode provided with a positive electrode collector and positive electrode active material which is held by the positive electrode collector and can occlude/emit a metal ion, a negative electrode provided with a negative electrode collector and negative electrode active material which is held by the negative electrode collector and can occlude/emit the metal ion, a minutely porous separator held between the positive electrode and the negative electrode and an organic electrolyte are provided, and a range of operating voltage is equivalent to a range from below 2 V to 4 V or more.