A battery pack comprising: (a) a battery module array having battery modules (210 and 220) arranged in two or more rows, each of the battery modules (210 and 220) having a structure in which battery cells or unit modules (212, 214 and 216), each having two or more battery cells mounted therein, are stacked in a state in which the battery cells or the unit modules (212, 214 and 216) are erected in an upside-down fashion; (b) a pair of side support members comprising a front support member and a rear support member that support the front and rear of the battery module array, respectively, in a state in which the pair of side support members are in close contact with outermost unit modules (212, 214 and 216) in the battery modules (210 and 220) in the battery module array; (c) lower end support members coupled to lower ends of the side support members to support a lower end of the battery module array; (d) two or more first upper mounting members coupled to upper ends of the side support members and lower ends of the battery modules (210 and 220) erected in an upside-down fashion, each of the first upper mounting members being fastened at one end thereof to an external device and having the shape of an angular pipe that is angular in vertical section; (e) a second upper mounting member coupled to upper ends of the first upper mounting members such that the second upper mounting member intersects the first upper mounting members at right angles, the second upper mounting member being fastened at opposite ends (502 and 504) thereof to the external device and having the shape of an angular pipe that is angular in vertical section; and (f) a rear mounting member disposed at the rear of the battery module array, the rear mounting member being fastened at opposite ends (502 and 504) thereof to the external device and having the shape of an angular pipe that is angular in vertical section. wherein each of the side support members comprises: a main body that is in contact with the outermost unit modules (212, 214 and 216) in the battery modules (210 and 220) in the battery module array; an upper end wall and a lower end wall, each being shaped to protrude outwardly from the perimeter of the main body; and a pair of side walls. (a) a battery module array having battery modules (210 and 220) arranged in two or more rows, each of the battery modules (210 and 220) having a structure in which battery cells or unit modules (212, 214 and 216), each having two or more battery cells mounted therein, are stacked in a state in which the battery cells or the unit modules (212, 214 and 216) are erected in an upside-down fashion; (b) a pair of side support members comprising a front support member and a rear support member that support the front and rear of the battery module array, respectively, in a state in which the pair of side support members are in close contact with outermost unit modules (212, 214 and 216) in the battery modules (210 and 220) in the battery module array; (c) lower end support members coupled to lower ends of the side support members to support a lower end of the battery module array; (d) two or more first upper mounting members coupled to upper ends of the side support members and lower ends of the battery modules (210 and 220) erected in an upside-down fashion, each of the first upper mounting members being fastened at one end thereof to an external device and having the shape of an angular pipe that is angular in vertical section; (e) a second upper mounting member coupled to upper ends of the first upper mounting members such that the second upper mounting member intersects the first upper mounting members at right angles, the second upper mounting member being fastened at opposite ends (502 and 504) thereof to the external device and having the shape of an angular pipe that is angular in vertical section; and (f) a rear mounting member disposed at the rear of the battery module array, the rear mounting member being fastened at opposite ends (502 and 504) thereof to the external device and having the shape of an angular pipe that is angular in vertical section. a main body that is in contact with the outermost unit modules (212, 214 and 216) in the battery modules (210 and 220) in the battery module array; an upper end wall and a lower end wall, each being shaped to protrude outwardly from the perimeter of the main body; and a pair of side walls.