Disclosed herein is an overcharge protection device including a diagnostic function. The overcharge protection device comprises: a mechanical overcharge protection device, connected between the relay actuation coil of a main relay and a ground, for disconnecting the connection between the relay actuation coil and the ground when the degree of swelling of a battery is equal to or greater than a predetermined value; and a control unit for diagnosing whether the mechanical overcharge protection device is operating or has failed by detecting the voltage between the two ends of the mechanical overcharge protection device. Accordingly, because whether the mechanical overcharge protection device is operating or has failed may be diagnosed accurately, it is possible to detect in advance that the mechanical overcharge protection device may not be operating normally due to the defects or failure thereof whereby a fail-safe overcharge protection device may be embodied and safety may be improved.