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 a lateral direction, a protection circuit module (PCM) connected to an upper end of the battery cell array to control an operation of the battery pack, a pack case in which the battery cell array and the protection circuit module are disposed, and a plate-shaped reinforcing member mounted between the pack case and the battery cell array to increase mechanical strength of the pack case.