Battery System and Ground Fault Detecting Device

A first battery pack (252-1) comprises a battery module (314) which includes a plurality of battery cells (310) connected in series between a positive terminal (352) and a negative terminal (353), ground-fault resistances (45) each of which appears between a grounding point (23) and an inter-battery connection point (311) between adjoining ones of the battery cells (310), a booster unit (11) which is provided to the positive terminal (352) and outputs boosted electric potential acquired by boosting high electric potential of the battery module (314), a protective resistance (25) and a ground fault detecting resistance (27) which are provided in series between a third connection point (19) (on the output side of the booster unit (11)) and the grounding point (23), and a ground fault detecting unit (41) which detects the presence/absence of a ground fault in the battery module (314) based on the result of comparison between preset threshold electric potential (Vref) and electric potential of an inter-resistance connection point (29) between the protective resistance (25) and the ground fault detecting resistance (27).