Provided is a battery pack comprising: a battery cell arranging body having two or more battery cells transversely arranged thereon, each of the battery cells being obtained by sealing an electrode assembly with a cathode/separating membrane/anode structure inside a battery case along with a liquid electrolyte; a protection circuit module (PCM) connected to the upper end part of the battery cell arranging body so as to control the operation of the battery pack; a pack case having the battery cell arranging body and the protection circuit module loaded thereon; and a spacer, mounted between the pack case and the battery cell arranging body, for securing space for the increase in the width of the battery cell arranging body during charge/discharge.