A secondary battery system comprising: a secondary battery having an electrode containing an active material into and from which a charge carrier is reversibly incorporated and removed; a monitoring unit including a voltage sensor, a current sensor and a temperature sensor; and a control unit configured to estimate a stress generated in the active material as the charge carrier is incorporated into or removed from the active material, characterized in that the control unit is configured to calculate an estimated value of the stress from a difference obtained by subtracting a reference charge carrier amount from an amount of the charge carrier contained in the active material, using a first linear relationship between the difference and the stress, and determining the estimated value as yield stress of the active material when a magnitude of the estimated value is larger than a magnitude of the yield stress of the active material, calculate an estimated OCV based on measurement values obtained by the monitoring unit, execute an SOC estimating process of estimating the SOC of the secondary battery, and calculate an amount of change of an OCV from the stress, using a second linear relationship between the amount of change of the OCV caused by the stress, relative to the OCV in a case where the stress is a reference stress, and the estimated stress, in the SOC estimating process; and estimate the SOC of the secondary battery from the amount of change of the OCV, by referring to a first correspondence relationship between the OCV and the SOC of the secondary battery when the stress is the reference stress. a secondary battery having an electrode containing an active material into and from which a charge carrier is reversibly incorporated and removed; a monitoring unit including a voltage sensor, a current sensor and a temperature sensor; and a control unit configured to estimate a stress generated in the active material as the charge carrier is incorporated into or removed from the active material, characterized in that the control unit is configured to calculate an estimated value of the stress from a difference obtained by subtracting a reference charge carrier amount from an amount of the charge carrier contained in the active material, using a first linear relationship between the difference and the stress, and determining the estimated value as yield stress of the active material when a magnitude of the estimated value is larger than a magnitude of the yield stress of the active material, calculate an estimated OCV based on measurement values obtained by the monitoring unit, execute an SOC estimating process of estimating the SOC of the secondary battery, and calculate an amount of change of an OCV from the stress, using a second linear relationship between the amount of change of the OCV caused by the stress, relative to the OCV in a case where the stress is a reference stress, and the estimated stress, in the SOC estimating process; and estimate the SOC of the secondary battery from the amount of change of the OCV, by referring to a first correspondence relationship between the OCV and the SOC of the secondary battery when the stress is the reference stress.