A battery pack has a base plate including one surface on which two or more battery module assemblies respectively having a plurality of battery cells arranged therein are mounted in a separated state from each other with a predetermined interval therebetween. A cover member battery is coupled at one surface of the base plate in a state that the module assemblies are incorporated therein. A reinforcement supporting member is positioned at a separation part between the battery module assemblies while supporting a mounting state of the cover member for the base plate.