There is provided a state-of-charge estimation method, a state-of-charge estimation device, and a secondary-battery power system that may quickly and stably determine the convergence value of an adjustment parameter of a voltage characteristic formula that may approximate change over time of an open-circuit voltage of a secondary battery with high precision by appropriately setting the initial value of the adjustment parameter. At step S14, a selected voltage measurement values V1, VMbi ((i=1 to (n−1)), and VMm are used to calculate an initial value A0i (i=1 to n) of an adjustment parameter Ai (i=1 to n). In addition, at step S15, an integer string bi (i=1 to (n−1)) and a real number C are used to calculate an initial value B01 (i=1 to n) of an adjustment parameter Bi (i=1 to n).