A battery state estimating apparatus includes an updating unit. The updating unit updates a charge-transfer impedance model in a battery model of a secondary battery, which is a series connection of a DC resistance model, a charge-transfer impedance model, and an diffusion impedance model, using the amount of change in a measurement value of a current flowing through the secondary such that a first relationship between the current flowing through the secondary battery and a voltage across the charge-transfer resistance approaches a second relationship between an actual value of the current and an actual value of the voltage across the charge-transfer resistance. The first relationship between the current flowing through the secondary battery and the voltage across the charge-transfer resistance is defined based on the Butler-Volmer equation. A state estimator estimates a state of the secondary battery based on the battery model including the updated charge-transfer impedance model.