A battery pack comprising: a battery module array having battery modules (210 and 220) arranged in two or more rows in a state in which the battery modules (210 and 220) are erected in an upside-down fashion, each of the battery modules (210 and 220) being formed by stacking battery cells or unit modules (212, 214 and 216), each having two or more battery cells mounted therein, in a lateral direction with respect to electrode terminals of the battery cells; a front support member and a rear support member that are a pair of side support members to 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; lower end support members coupled to lower ends of the side support members to support a lower end of the battery module array; and mounting members coupled to an upper end of the battery module array and fastened to an external device such that an upper part of the battery pack is mounted to the external device, characterized in that 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 battery module array having battery modules (210 and 220) arranged in two or more rows in a state in which the battery modules (210 and 220) are erected in an upside-down fashion, each of the battery modules (210 and 220) being formed by stacking battery cells or unit modules (212, 214 and 216), each having two or more battery cells mounted therein, in a lateral direction with respect to electrode terminals of the battery cells; a front support member and a rear support member that are a pair of side support members to 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; lower end support members coupled to lower ends of the side support members to support a lower end of the battery module array; and mounting members coupled to an upper end of the battery module array and fastened to an external device such that an upper part of the battery pack is mounted to the external device, characterized in that 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 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.