A method for estimating a state of charge of a valve regulated lead-acid battery includes; an overcharge amount identification step of identifying an overcharge amount when the lead-acid battery is overcharged after an arbitrary reference time; an open circuit voltage acquisition step of acquiring an open circuit voltage of the lead-acid battery after the reference time; and a state-of-charge estimation step of estimating a state of charge of the lead-acid battery based on the acquired open circuit voltage and a correlation between an open circuit voltage and state of charge in which a rate of change of the state of charge with respect to the open circuit voltage is smaller as the overcharge amount from the reference time until an acquisition time of the open circuit voltage is larger.