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 a flow space (‘inlet duct’) extending from the coolant inlet port to the battery module and another flow space (Outlet duct’) extending from the battery module to the coolant outlet port, the inlet duct having a vertical sectional area less than that of the outlet duct.