Electricity storage device and electrode used therein

This electricity storage device has: an electricity storage element; an electrolyte with which the electricity storage element is impregnated; and an outer housing having the electricity storage element and the electrolyte housed therein. The electricity storage element has a positive electrode, a negative electrode facing the positive electrode, and a separator that is disposed between the positive electrode and the negative electrode. The negative electrode contains a carbon material, and lithium stored in the carbon material. The negative electrode contains, at a predetermined potential, a larger quantity of lithium compared with a quantity of lithium stored in a case where the carbon material in a state prior to storing lithium is impregnated with the electrolyte containing lithium ions, and potential is set at the predetermined potential.