Disclosed is a battery pack having a plurality of battery cells or unit modules (‘unit cells’), which can be charged and discharged, mounted in a pack case, wherein the unit cells are stacked in a direction (a Z direction) in which the unit cells are sequentially stacked in parallel to a ground in a state in which a spacing distance for coolant flow is provided between the respective unit cells to constitute a battery module, two or more battery modules are arranged in a horizontal direction (an X direction) on the plane with respect to a coolant introduction direction in which a coolant is introduced through a coolant inlet port in a state in which flow of the coolant between the battery modules is restrained to constitute a battery module group.