A secondary battery system includes a secondary battery having a positive electrode including a positive electrode active substance and a negative electrode including first and second negative electrode active substances, and a control device that estimates an internal state of the secondary battery based on an active substance model of the secondary battery. The control device calculates a charge carrier amount in the first negative electrode active substance based on a first active substance model, under a condition that the first and second negative electrode active substances are at the same potential, calculates an amount of change in open circuit potential of the first negative electrode active substance based on surface stress of the first negative electrode active substance, and calculates an open circuit potential of the negative electrode from the open circuit potential and the amount of change in open circuit potential of the first negative electrode active substance.