A battery pack , comprising: a mounting structure including a first frame having a body portion with a plate of a lying-down form and sidewalls bent from both ends of the body portion and extending upward to form an inner space above the plate, and a second frame coupled to a lower portion of the first frame and having a body portion with a plate of a lying-down form and sidewalls bent from both ends of the body portion and extending downward to form an inner space below the plate; and at least two battery modules respectively accommodated in the inner spaces of the first frame and the second frame and including a plurality of cylindrical battery cells and a module case having a plurality of hollow tubes into which the plurality of cylindrical battery cells standing upright in an upper and lower direction are inserted and a connection portion configured to connect the plurality of hollow tubes to each other, wherein the plurality of cylindrical battery cells are arranged in a plurality of columns and rows, the plurality of cylindrical battery cells are arranged in a zigzag form along with cylindrical battery cells located in another adjacent row, and the connection portion extends in a diagonal direction to connect one hollow tube to another hollow tube located in another row, wherein the connection portion has a plate shape of which one end is connected to one hollow tube and the other end is connected to another hollow tube, wherein a vulnerable part is formed at the connection portion so as to be ruptured or deformed by an external impact of the battery module, wherein the module case includes an upper case having a plurality of upper hollow tubes configured to accommodate an upper portion of the plurality of standing cylindrical battery cells, and a lower case spaced apart from the upper case in a lower direction and having a plurality of lower hollow tubes configured to accommodate a lower portion of the plurality of standing cylindrical battery cells. a mounting structure including a first frame having a body portion with a plate of a lying-down form and sidewalls bent from both ends of the body portion and extending upward to form an inner space above the plate, and a second frame coupled to a lower portion of the first frame and having a body portion with a plate of a lying-down form and sidewalls bent from both ends of the body portion and extending downward to form an inner space below the plate; and at least two battery modules respectively accommodated in the inner spaces of the first frame and the second frame and including a plurality of cylindrical battery cells and a module case having a plurality of hollow tubes into which the plurality of cylindrical battery cells standing upright in an upper and lower direction are inserted and a connection portion configured to connect the plurality of hollow tubes to each other, wherein the plurality of cylindrical battery cells are arranged in a plurality of columns and rows, the plurality of cylindrical battery cells are arranged in a zigzag form along with cylindrical battery cells located in another adjacent row, and the connection portion extends in a diagonal direction to connect one hollow tube to another hollow tube located in another row, wherein the connection portion has a plate shape of which one end is connected to one hollow tube and the other end is connected to another hollow tube, wherein a vulnerable part is formed at the connection portion so as to be ruptured or deformed by an external impact of the battery module, wherein the module case includes an upper case having a plurality of upper hollow tubes configured to accommodate an upper portion of the plurality of standing cylindrical battery cells, and a lower case spaced apart from the upper case in a lower direction and having a plurality of lower hollow tubes configured to accommodate a lower portion of the plurality of standing cylindrical battery cells. <EOS>