The present invention provides a battery pack in which multiple battery modules are connected in series to one another in a tightly or adjacently stacked manner. The battery pack comprises: a cut-off unit which is fixed to maintain the stacked manner of the battery modules despite changes in volume during charging/discharging, and which is connected in series to an electrical connection circuit of the battery modules; a fixing member for fixing the following circuit breaker on at least one outer surface of the battery pack; and the circuit breaker which is configured to be electrified when outer surfaces of the battery modules expand larger than a reference volume, and which is connected in series to the electrical connection circuit so as to disconnect the cut-off unit when electrified by the expansion of the battery modules.