a plurality of secondary battery cells;

a cell accommodating unit equipped with at least one of the secondary battery cells in a procumbent state;

a cover unit coupled to the cell accommodating unit and covering an opening of the cell accommodating unit for accommodating the secondary battery cells; and

a heat sink unit provided in the cell accommodating unit and the cover unit and in contact with a bottom portion and a top portion of the secondary battery cells to externally release heat generated by the secondary battery cells,

wherein the cell accommodating unit comprises:

a lower plate member in which secondary battery cells are seated and in contact with a side surface portion of a lowermost secondary battery cell;

an upper plate member disposed to face the lower plate member; and

a middle plate member coupled to connect the lower plate member and the upper plate member and in contact with the bottom portion of the secondary battery cells;

wherein the middle plate member is bonded to a central portion of the upper plate member and a central portion of the lower plate member separating the cell accommodating unit into two accommodating sub-units, each accommodating a different stack of secondary battery cells,

wherein both sides of the middle plate member contact the bottom portion of the secondary battery cells,

wherein the cover unit includes a side cover member disposed to face the middle plate member and coupled to corner portions of the upper plate member and the lower plate member,

wherein the side cover member includes a support tab protruding into the remaining area of the top of the secondary battery cell adjacent to the area in which the sealing portion of the secondary battery cell is folded and disposed,

wherein the heat sink unit comprises a center heat sink provided in the middle plate member and releasing heat generated by the bottom portion of the secondary battery cells externally and a side heat sink provided in the side cover member and releasing heat generated by the top portion of the secondary battery cells externally, and

wherein the side heat sink is provided to extend to the support tab.