A method for recovering capacity of a lithium ion battery, the battery including a positive electrode containing a first active material and a second active material which is base in terms of electrode potential as compared with the first active material, a negative electrode, an electrolytic solution, and a lithium ion replenishing electrode for releasing lithium ions, the method comprising: a determination step of comparing charge or discharge characteristics in an initial stage of the lithium ion battery with charge or discharge characteristics at a time of determination after a lapse of a predetermined period from the initial stage to determine a cause of degradation of the lithium ion battery, wherein if the comparison between charge or discharge capacity in the initial stage and charge or discharge capacity at the time of determination shows that the electrode potential corresponding to the first active material at the time of determination is not found to be lower than the electrode potential corresponding to the second active material in the initial stage and that the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the first active material in the initial stage is nearly the same as the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the first active material at the time of determination, and that the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the second active material at the time of determination is shorter than the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the second active material in the initial stage, it is determined that the cause of degradation is a decrease in lithium ions; a calculation step of calculating an amount of the decrease in lithium ions based on a difference between the charge or discharge capacity at the time of determination and the charge or discharge capacity in the initial stage when the electrode potential corresponding to the second active material is held as the potential difference; and a replenishment step of connecting the lithium ion replenishing electrode to the positive electrode or the negative electrode to release lithium ions corresponding to the amount of the decrease from the lithium ion replenishing electrode, thereby replenishing the lithium ion battery with lithium ions. a determination step of comparing charge or discharge characteristics in an initial stage of the lithium ion battery with charge or discharge characteristics at a time of determination after a lapse of a predetermined period from the initial stage to determine a cause of degradation of the lithium ion battery, wherein if the comparison between charge or discharge capacity in the initial stage and charge or discharge capacity at the time of determination shows that the electrode potential corresponding to the first active material at the time of determination is not found to be lower than the electrode potential corresponding to the second active material in the initial stage and that the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the first active material in the initial stage is nearly the same as the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the first active material at the time of determination, and that the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the second active material at the time of determination is shorter than the length of a region where the voltage during charging or discharging is held at the electrode potential corresponding to the second active material in the initial stage, it is determined that the cause of degradation is a decrease in lithium ions; a calculation step of calculating an amount of the decrease in lithium ions based on a difference between the charge or discharge capacity at the time of determination and the charge or discharge capacity in the initial stage when the electrode potential corresponding to the second active material is held as the potential difference; and a replenishment step of connecting the lithium ion replenishing electrode to the positive electrode or the negative electrode to release lithium ions corresponding to the amount of the decrease from the lithium ion replenishing electrode, thereby replenishing the lithium ion battery with lithium ions.