A protection device for an assembled cell including battery modules connected in series includes protection units each of which is configured to operate using a corresponding battery module as a power supply and a management unit connected to the protection units in common, the protection unit includes a measuring unit configured to measure its own consumption current to obtain measurement data, a communication unit which transmits the measurement data to a management unit and receives setting data from the management unit, a discharging unit which discharges the corresponding battery module, and a control unit configured to control the discharging unit based on the measurement data and the setting data so that a total of the consumption current and a discharging current by the discharging unit is of a constant value among the protection units.