Disclosed herein is a terminal-connecting device including an electrically insulating hollow connecting device body having an open upper end, and an electrically insulating cover coupled to the open upper end of the connecting device body. The terminal-connecting device is constructed such that a plate-shaped conductive bus bar is mounted to the lower end of the connecting device body, two or more terminal insertion holes are formed in the lower end of the connecting device body and the bus bar, the terminal insertion holes formed in the lower end of the connecting device body communicating with the terminal insertion holes formed in the bus bar, and a connecting member insertion slit is formed in one side of connecting device body for allowing further connection of the corresponding electrode terminal to an external connecting member, as occasion demands. The electrical connection between the electrode terminals and the electrical connection between the external connecting member and the corresponding electrode terminal can be easily and conveniently accomplished through the use of the terminal-connecting device according to the present invention. Also, the terminal connection part is isolated from the outside, whereby the occurrence of short circuits and the occurrence of corrosion due to moisture are effectively prevented. The terminal-connecting device according to the present invention is preferably used in manufacturing a medium- or large-sized battery system, especially, for electric vehicles and hybrid electric vehicles.