A battery pack , which includes at least one battery module and a pack case for packaging the at least one battery module, the pack case of the battery pack comprising a case base , a vase body and a case cover , wherein: - the case base forms a bottom of the pack case and is configured to support the at least one battery module, wherein the case base includes: - a base mount mounted to the at least one battery module; and - drainage holes provided at both sides of the base mount and formed to have a step from the base mount; - the case body is coupled to the case base to accommodate the at least one battery module, wherein the case body includes: - a first inner case including four fixing support bars configured to fix the at least one battery module at four corners of the at least one battery module; - an outer case configured to surround the first inner case and form an accommodation space for accommodating the at least one battery module, wherein the outer case is mounted to the first inner case by sliding along the vertical direction of the at least one battery modules ; and - a second inner case mounted to an upper side of the outer case and coupled to the case cover , - the case cover is coupled to the case body to cover the case body, wherein the case base and the case body are sealed to each other and the case body and the case cover are sealed to each other, respectively, by means of a sealant member along a rim thereof, and wherein the second inner case is mutually sealed to the outer case and to the case cover by means of the sealant member and wherein the outer case is mutually sealed to the case base by means of the sealant member. - the case base forms a bottom of the pack case and is configured to support the at least one battery module, wherein the case base includes: - a base mount mounted to the at least one battery module; and - drainage holes provided at both sides of the base mount and formed to have a step from the base mount; - a base mount mounted to the at least one battery module; and - drainage holes provided at both sides of the base mount and formed to have a step from the base mount; - the case body is coupled to the case base to accommodate the at least one battery module, wherein the case body includes: - a first inner case including four fixing support bars configured to fix the at least one battery module at four corners of the at least one battery module; - an outer case configured to surround the first inner case and form an accommodation space for accommodating the at least one battery module, wherein the outer case is mounted to the first inner case by sliding along the vertical direction of the at least one battery modules ; and - a second inner case mounted to an upper side of the outer case and coupled to the case cover , - a first inner case including four fixing support bars configured to fix the at least one battery module at four corners of the at least one battery module; - an outer case configured to surround the first inner case and form an accommodation space for accommodating the at least one battery module, wherein the outer case is mounted to the first inner case by sliding along the vertical direction of the at least one battery modules ; and - a second inner case mounted to an upper side of the outer case and coupled to the case cover , - the case cover is coupled to the case body to cover the case body, <EOS>