Disclosed herein is a middle- or large-sized battery pack case in which a battery module having a plurality of stacked battery cells, which can be charged and discharged, is mounted, wherein the battery pack case is provided with a coolant inlet port and a coolant outlet port, which are disposed such that a coolant for cooling the battery cells can flow from one side to the other side of the battery module in the direction perpendicular to the stacking direction of the battery cells, and the battery pack case is further provided with beads formed in a concavo-convex shape for improving the structural stability of the battery pack case against an external force, the beads being constructed in an outwardly-protruding structure to allow the coolant to be uniformly introduced into the battery module from the coolant inlet port in the advancing direction of a fluid in a flow space ("inlet duct") defined between the coolant inlet port and the battery module.