Disclosed herein is a middle or large-sized battery pack case in which a battery module having a plurality of stacked battery cells or unit modules (‘unit cells’), which can be charged and discharged, is mounted, wherein the battery pack case is provided at an upper part and a lower part thereof with a coolant inlet port and a coolant outlet port, respectively, which are directed in opposite directions such that a coolant to cool the unit cells can flow from one side to the other side of the battery module in the direction perpendicular to the stacking direction of the unit cells, the battery pack case is further provided with a flow space (‘coolant introduction part’) extending from the coolant inlet port to the battery module and another flow space (‘coolant discharge part’) extending from the battery module to the coolant outlet port, an upper end inside of the coolant introduction part facing the top of the battery module is configured so that the distance between the upper end inside of the coolant introduction part and the top of the battery module decreases toward an end of the battery pack case opposite to the coolant inlet port, and a downwardly projected portion to guide the coolant to the battery module is formed at the upper end inside of the coolant introduction part so that the rugged portion extends a predetermined distance from the end of the battery pack case opposite to the coolant inlet port.