Disclosed herein is a middle- or large-sized battery pack having a plurality of battery cells, which can be charged and discharged, stacked one on another with high density and electrically connected with each other. When the battery cells swell due to abnormal operation of the battery pack or degradation of the battery pack caused by the charge and discharge of the battery cells for a long period of time, stress is concentrated on a predetermined region of the battery pack due to the change of thickness of the swelling battery cells, whereby the physical change of the battery pack is caused, and the disconnection of an electrically connecting member of the battery pack is mechanically accomplished by the physical change of the battery pack. Consequently, the battery pack according to the present invention provides high safety.