Relay welding detector of battery system and battery system which uses the detector

Provided is a battery system which can ensure high safety even when the battery system is constructed using a mechanical relay. The battery system includes: a battery module 30 where a plurality of battery cells 20 are connected to each other in series; a switching circuit 400 which is connected to the battery module 30 in series; a control unit 230 which controls the switching circuit 400; and a welding diagnosis circuit 410 which diagnoses welding of the switching circuit 400, wherein the switching circuit 400 is constituted by connecting a first relay switch 401 and a second relay switch 402 in series, and the welding diagnosis circuit 410 includes a voltage measuring device 415 which measures a voltage between the first relay switch 401 and the second relay switch 402.