Disclosed is a lifetime estimating method for a lithium secondary cell, in which the voltage of the lithium secondary cell charged and discharged by different cycle numbers as a charging-discharging cycle has elapsed is detected at least two times at an open-circuit time after the discharge. At least two of the individual voltage values are plotted against the individual cycle numbers. An arc passing through the individual plot points is drawn so that the lifetime of the lithium secondary cell is estimated on the basis of the size of that arc. The progress of the deterioration can be suppressed by controlling the charge and discharge of the lithium secondary cell on the basis of that lifetime estimation.