A middle- or large-sized battery pack comprising: a module assembly constructed in a structure in which a plurality of battery modules, each of which includes a plurality of battery cells or unit modules mounted in a module case while the battery cells or unit modules are connected in series to each other, are arranged such that the battery modules are disposed in contact with each other in the lateral direction, and comprising a coolant flow channel; a plurality of support members for supporting opposite sides and the bottom of the module assembly and maintaining the arrangement state of the module assembly; and a pack housing for surrounding the module assembly and the support members, wherein the battery pack is constructed in a cooling structure in which a coolant is introduced through one side upper end of the module assembly along a hermetically sealed space defined by the support members, flows vertically through the module assembly, and is discharged through the other side lower end of the module assembly, wherein the support members include a pair of side support members for preventing the coolant from leaking to the outside while surrounding sides of the outermost battery modules of the module assembly, wherein each of the side support members is generally constructed in a plate-shaped structure including a plate-shaped body, a pair of bottom support members for supporting front and rear lower ends of the module assembly while spacing the module assembly from the bottom by a predetermined height, the bottom support members being coupled to the side support members, and at least one top connection member (141, 142, 143) connected to the side support members for guiding the coolant to the module assembly from the top of the module assembly, and characterized in that an insulation member is mounted to the inside of the plate-shaped body where the plate-shaped body is disposed in contact with the module assembly for preventing the leakage of the coolant, securing the uniformity in temperature of the module assembly, and preventing the occurrence of a short circuit, at the top of the module assembly are formed a plurality of through-holes for each battery module, and the at least one top connection member (141, 142, 143) connected to the side support members includes two or more top connection members (141, 142, 143), and, among the top connection members (141, 142, 143), the top connection member located near the front of the module assembly has a height sufficient to come into tight contact with the pack housing in the longitudinal direction thereof, said top connection member being configured to press the front upper end of the module assembly and to be brought, at the same time, into contact with the inside top of the pack housing, with the result that, in use, a coolant, which is introduced into the rear upper end of the module assembly from being discharged through the front of the module assembly, and that, in use, the coolant is guided such that the coolant flows downward vertically through the module assembly after the coolant is being introduced into the module assembly through the through-holes, and is being discharged through the front lower end of the module assembly.