Disclosed is a battery pack, which includes a plurality of unit battery modules, each unit battery module having a plurality of battery cells accommodated in an inner space thereof, the plurality of unit battery modules being structurally connected successively in one direction; and a pack case configured to cover the plurality of unit battery modules, wherein each unit battery module has two side plates forming opposite side surfaces thereof, and wherein one of the side plates of any one of the unit battery modules and one of the side plates of another one of the unit battery modules adjacent thereto are engaged with each other to form a single common wall.