Disclosed herein is a battery pack including a battery cell array including two or more battery cells, each of which has an electrode assembly of a cathode/separator/anode structure disposed in a battery case together with an electrolyte in a sealed state, arranged in the lateral direction, a protection circuit module (PCM) connected to the upper end of the battery cell array to control the operation of the battery pack, a pack case in which the battery cell array and the protection circuit module are disposed, and a spacer mounted between the pack case and the battery cell array to provide a space accommodating the increase in thickness of the battery cell array during charge and discharge of the battery pack.