An electric power supply system for supplying a load device with driving electric power includes an electric power storage device and an ECU. The electric power storage device includes a CID configured to interrupt an electrical conduction path of the electric power storage device in response to the electric power storage device having an internal pressure exceeding a rated value. The controller calculates a voltage variation length corresponding to an integral of an amount by which a voltage applied to the load device varies in magnitude and a current variation length corresponding to an integral of an amount by which a current input/output to/from the electric power storage device varies in magnitude for each sampling period for a predetermined period of time. Then, the ECU determines whether or not the CID has been operated, based on the voltage variation length and the current variation length.