Disclosed herein is a battery pack configured to have a structure including a plate-shaped battery cell having electrode terminals formed at one side thereof including a sealed surplus part and a protection circuit module (PCM) mounted at the sealed surplus part, wherein each of the electrode terminals of the battery cell is made of a plate-shaped conductive member, the PCM includes a protection circuit board (PCB), a safety element electrically connected between one of the electrode terminals formed at one side of the battery cell and the PCB or loaded on the PCB, an external input and output terminal electrically connected to a protection circuit of the PCB, and an electrically insulative module case in which the PCB and the safety element are mounted in a state in which the external input and output terminal extends outside, the module case is configured to have a structure in which two outer surfaces of the module case are open outward to form a first open surface and a second open surface, the module case is provided at an interior thereof with a PCB receiving part, and the PCM is loaded on the sealed surplus part of the battery cell in a state in which the PCM is received in the module case such that the PCM is electrically connected to the electrode terminals of the battery cell.