A battery pack case (100, 200, 300, 400, 500, 600, 900) configured to receive a battery cell stack constituted by a plurality of stacked battery cells therein, the battery pack case comprising: a first case (110, 210, 310, 410, 510, 610, 910) configured to allow the battery cell stack to be mounted therein; and a second case (120, 220, 320, 420, 520, 620, 920) coupled to the first case (110, 210, 310, 410, 510, 610, 910), the second case (120, 220, 320, 420, 520, 620, 920) being configured to cover the battery cell stack, wherein the first case (110, 210, 310, 410, 510, 610, 910) comprises a first flat panel portion (111, 211, 311, 411, 511, 811, 911) and a first side portion (112, 212, 312, 412, 512, 812, 912), wherein the second case (120, 220, 320, 420, 520, 620, 920) comprises a second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921) and a second side portion (122, 222, 322, 422, 522, 622, 722, 822, 922), wherein a thickness of the second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921) of the second case (120, 220, 320, 420, 520, 620, 920) is less than a thickness of the second side portion (122, 222, 322, 422, 522, 622, 722, 822, 922) of the second case (120, 220, 320, 420, 520, 620, 920), wherein the second case (120, 220, 320, 420, 520, 620, 920) comprises two second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) extending, in a direction perpendicular to the second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921), and from opposite outer peripheries of the second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921) in a longitudinal direction thereof, wherein each of the second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) is configured to have a structure including a step at an outer surface of the second side portion (122, 222, 322, 422, 522, 622, 722, 822, 922) such that a thickness of an end thereof is reduced, characterized in that a stepped portion (423, 523, 623, 823) of each of the second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) includes a vertical hole (627, 727) formed therethrough in a height direction thereof and/or in that the stepped portion (423, 523, 623, 823) of each of the second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) includes a horizontal hole formed therethrough in a horizontal direction thereof. a first case (110, 210, 310, 410, 510, 610, 910) configured to allow the battery cell stack to be mounted therein; and a second case (120, 220, 320, 420, 520, 620, 920) coupled to the first case (110, 210, 310, 410, 510, 610, 910), the second case (120, 220, 320, 420, 520, 620, 920) being configured to cover the battery cell stack, wherein the first case (110, 210, 310, 410, 510, 610, 910) comprises a first flat panel portion (111, 211, 311, 411, 511, 811, 911) and a first side portion (112, 212, 312, 412, 512, 812, 912), wherein the second case (120, 220, 320, 420, 520, 620, 920) comprises a second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921) and a second side portion (122, 222, 322, 422, 522, 622, 722, 822, 922), wherein a thickness of the second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921) of the second case (120, 220, 320, 420, 520, 620, 920) is less than a thickness of the second side portion (122, 222, 322, 422, 522, 622, 722, 822, 922) of the second case (120, 220, 320, 420, 520, 620, 920), wherein the second case (120, 220, 320, 420, 520, 620, 920) comprises two second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) extending, in a direction perpendicular to the second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921), and from opposite outer peripheries of the second flat panel portion (121, 221, 321, 421, 521, 621, 821, 921) in a longitudinal direction thereof, wherein each of the second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) is configured to have a structure including a step at an outer surface of the second side portion (122, 222, 322, 422, 522, 622, 722, 822, 922) such that a thickness of an end thereof is reduced, characterized in that a stepped portion (423, 523, 623, 823) of each of the second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) includes a vertical hole (627, 727) formed therethrough in a height direction thereof and/or in that the stepped portion (423, 523, 623, 823) of each of the second side portions (122, 222, 322, 422, 522, 622, 722, 822, 922) includes a horizontal hole formed therethrough in a horizontal direction thereof.