In a power storage system equipped with a secondary battery, a control system of the secondary battery includes a unit which detects an index indicating a degradation state of a positive electrode, a unit which detects an index indicating a degradation state of a negative electrode, a unit which calculates a difference between the index of the positive electrode and the index of the negative electrode, a unit which determines a sign of the difference, and a unit which changes an operation condition of the secondary battery according to the sign. With this configuration, in the control of the secondary battery, it is possible to avoid a life estimation accuracy from being decreased while using a processor which has a high-speed processing performance and is inexpensive.