Discussed is battery pack case configured to receive a battery cell stack constituted by a plurality of battery cells stacked therein, the battery pack case including a first case configured to allow the battery cell stack to be mounted therein and a second case coupled to the first case, the second case being configured to cover the battery cell stack, wherein the first case includes a first flat panel portion and a first side portion, wherein the second case includes a second flat panel portion and a second side portion, and wherein a thickness of the second flat panel portion of the second case is less than the thickness of the second side portion of the second case.