Provided is an internal resistance estimating method capable of estimating the internal resistance of a secondary battery, taking account of conditions when the secondary battery is being charged, without requiring progress information when the secondary battery is being used. The internal resistance estimating method for estimating the internal resistance of a secondary battery includes estimating an internal resistance Rd (Ts, Is, Va) on the basis of an internal resistance calculating formula, from an acquired internal resistance Rd (Ta, la, Va) of a battery, and estimating an internal resistance Rd (Ts, Is, Vs) from the estimated internal resistance Rd (Ts, Is, Va).