Cell Assembly Unit and Battery Pack Including the Same

A battery pack includes a cell assembly unit including a plurality of battery cells and a bus bar configured to electrically connect the plurality of battery cells, a frame unit accommodating the cell assembly unit, and a bracket unit fixing the cell assembly unit to the frame unit. The bracket unit and the frame unit form a flow path through which a gas is discharged. A reverse flow prevention member is disposed between the bus bar and the bracket unit. The reverse flow prevention member includes a plurality of gas permeation holes, each of which extends from one end facing the bus bar and leads to another end facing the flow path.