Discussed is a battery relay diagnostic device adapted to diagnose a battery system including battery packs connected and a charge and discharge controller connected between the battery packs and a load part and control charge and discharge of the battery packs, the battery packs including a battery cell module including battery cells and a switching unit including a first switch provided between a positive ((+)) terminal of the battery cell module and a load part and a second switch provided between a negative ((−)) terminal of the battery cell module and the load part, the battery relay diagnostic device including: a switching controller configured to control a current applied to the battery cell module by controlling on/off of the switching unit; and a diagnostic circuit configured to perform diagnosis on the first switch and the second switch based on an on/off state of the first switch and the second switch.