To appropriately evaluate the deteriorating state of a secondary battery in a state of being mounted to a moving body in view of a practical capacity thereof.

The deteriorating state diagnostic system for the secondary battery diagnoses the deteriorating state of the secondary battery RB supplying power to an electric motor for the startup of an engine by being mounted to the moving body with the engine started by an electric motor. A minimum voltage detecting operation detecting a minimum voltage during engine startup operation after a time when the voltage first becomes the maximum after the start of current-carrying to the electric motor in a temporal change of an output voltage of the battery RB is executed at the beginning of the start of use of the battery RB to learn an initial minimum voltage value. Then, the deteriorating state of the battery RB is diagnosed based on a degree of a decline of the minimum voltage detected by executing the minimum voltage detecting operation from the initial minimum voltage value.