Disclosed herein is a safety device mounted at one side of a battery pack including two or more battery cells or at least one battery module such that the safety device is first short-circuited when a needle type object penetrates the battery pack to secure safety of the battery pack, the safety device including a pair of conductive sheets spaced apart from each other, an electrically insulative housing to surround outsides of the conductive sheets excluding fronts of the conductive sheets in a state in which the conductive sheets are inserted and mounted in the housing, a sealing member to cover the fronts of the conductive sheets, and a connection member to connect the conductive sheets to a cathode and an anode of one of battery cells constituting the battery module.