A battery pack configured to have a structure comprising a plate-shaped battery cell having electrode terminals (110, 120) formed at one side thereof comprising a sealed surplus part and a protection circuit module mounted at the sealed surplus part, wherein each of the electrode terminals (110, 120) of the battery cell is made of a plate-shaped conductive member, the PCM comprises a protection circuit board, a safety element electrically connected between one of the electrode terminals (110, 120) 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 (110, 120) of the battery cell, characterized in that the plate-shaped battery cell is a pouch-shaped secondary battery having an electrode assembly received in a battery case made of a laminate sheet comprising a resin layer and a metal layer in a sealed state, and the module case is configured to have a tunnel structure in which the first open surface and the second open surface communicate with each other in a straight line, and the first open surface or the second open surface is oriented toward the electrode assembly of the battery cell in a state in which the module case is loaded on the sealed surplus part of the battery cell. each of the electrode terminals (110, 120) of the battery cell is made of a plate-shaped conductive member, the PCM comprises a protection circuit board, a safety element electrically connected between one of the electrode terminals (110, 120) 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 (110, 120) of the battery cell, characterized in that the plate-shaped battery cell is a pouch-shaped secondary battery having an electrode assembly received in a battery case made of a laminate sheet comprising a resin layer and a metal layer in a sealed state, and the module case is configured to have a tunnel structure in which the first open surface and the second open surface communicate with each other in a straight line, and the first open surface or the second open surface is oriented toward the electrode assembly of the battery cell in a state in which the module case is loaded on the sealed surplus part of the battery cell.