A battery pack configured such that a plurality of battery modules is connected to each other in series in a state in which the battery modules are in tight contact with each other or stacked adjacent to each other, the battery pack being fixed such that a stacked state of the battery modules is maintained even when volume of the battery modules is changed during charge and discharge of the battery modules, the battery pack comprising: a cut-off portion connected in series to an electrical connection circuit between the battery modules; a fixing member to fix a circuit breaker to at least one outer surface of the battery pack; and the circuit breaker configured to be electrically conducted when an outer surface of at least one battery module expands by a reference volume value or more, the circuit breaker being connected in series to the electrical connection circuit to short-circuit the cut-off portion when electric conduction is performed due to swelling of the battery modules, characterized in that the circuit breaker comprises: an electrically insulative housing fixed to the fixing member in position, the housing having a receiving part open at a top thereof; an electrically insulative cap mounted in the receiving part while being in contact with an side of one of the battery modules such that the cap can elastically move in response to the change in volume of the battery modules; an electrically conductive electric conduction connection member interlocked with the cap; and a conduction member comprising a cathode conduction part and an anode conduction part, each of which has one end located inside the receiving part such that one end comes into contact with the electric conduction connection member when the volume of the battery modules increases and the other end located outside the receiving part such that the other end is connected in series to the electrical connection circuit between the battery modules. a cut-off portion connected in series to an electrical connection circuit between the battery modules; a fixing member to fix a circuit breaker to at least one outer surface of the battery pack; and the circuit breaker configured to be electrically conducted when an outer surface of at least one battery module expands by a reference volume value or more, the circuit breaker being connected in series to the electrical connection circuit to short-circuit the cut-off portion when electric conduction is performed due to swelling of the battery modules, characterized in that the circuit breaker comprises: an electrically insulative housing fixed to the fixing member in position, the housing having a receiving part open at a top thereof; an electrically insulative cap mounted in the receiving part while being in contact with an side of one of the battery modules such that the cap can elastically move in response to the change in volume of the battery modules; an electrically conductive electric conduction connection member interlocked with the cap; and a conduction member comprising a cathode conduction part and an anode conduction part, each of which has one end located inside the receiving part such that one end comes into contact with the electric conduction connection member when the volume of the battery modules increases and the other end located outside the receiving part such that the other end is connected in series to the electrical connection circuit between the battery modules. an electrically insulative housing fixed to the fixing member in position, the housing having a receiving part open at a top thereof; an electrically insulative cap mounted in the receiving part while being in contact with an side of one of the battery modules such that the cap can elastically move in response to the change in volume of the battery modules; an electrically conductive electric conduction connection member interlocked with the cap; and a conduction member comprising a cathode conduction part and an anode conduction part, each of which has one end located inside the receiving part such that one end comes into contact with the electric conduction connection member when the volume of the battery modules increases and the other end located outside the receiving part such that the other end is connected in series to the electrical connection circuit between the battery modules.