A relay diagnosis apparatus for a parallel pack assembly including a first battery pack having a first battery module and a first positive relay and a second battery pack having a second battery module and a second positive relay includes a diagnosis unit including a diagnosis switch and a diagnosis resistor connected between first and second battery pack terminals, a first detector to detect a current of the first battery module, a second detector to detect a current of the second battery module, and a control unit to collect a first and second detection values from the first and second detectors, respectively, during a first diagnosis period in which the first and second positive relays are open and the diagnosis switch is closed. The control unit determines a stuck-closed fault of the first or second positive relays based on the first and second detection values and a first threshold.