Provided is a secondary battery system which can accurately detect a state of a secondary battery system (such as a secondary battery state and a secondary battery system failure). The secondary battery system (6) includes dV/dQ calculation means which calculates a dV/dQ value as a ratio of a change amount dV of a battery voltage V of a secondary battery (100) against a change amount dQ of an accumulation amount Q when the accumulation amount Q of the secondary battery (100) is changed. The secondary battery system (6) detects the state of the secondary battery system (6) by using the dV/dQ value.