Disclosed herein is a spacer used in a battery pack having a plurality of battery cells mounted in a pack case, wherein the spacer is configured to have a top shape corresponding to the inner shape of the pack case in vertical section and a bottom shape corresponding to the outer circumferential shape of the battery cells in vertical section so that the spacer can be mounted in a space defined between the battery cells and the pack case, grooves, in which electrical connection members are mounted, are formed at the top of the spacer, and the spacer includes at least one temperature detection member mounting part including a horizontal opening open in one direction or in opposite directions and a vertical opening communicating with the horizontal opening so that a temperature detection member is inserted from one side of the spacer and mounted in a space defined between the battery cells. In the spacer, the temperature detection member is inserted into the horizontal opening from one side of the spacer so that the temperature detection member can be fixed between the battery cells and the spacer, and therefore, a process of bonding the temperature detection member is omitted. Also, the electrical connection members for electrical connection between the battery cells and between the battery cells and a protection circuit member are mounted in the grooves so that the electrical connection members are fixed in an insulated state, thereby preventing the occurrence of a short circuit due to vibration or impact and simplifying an assembly process.