A curtain airbag capable of applying tension in a vertical direction to improve occupant ejection mitigation performance. The curtain airbag is housed in an upper portion of a side surface inside a vehicle cabin and deployed along the side surface. The curtain airbag includes: a main chamber having a protection area for the occupant, and receiving an inflation gas and being inflated along a side window of a vehicle; a first chamber that overlaps an upper edge of the side window of the vehicle, the first chamber extends by being deployed toward an upper side of the main chamber along the upper portion of the side surface inside the vehicle cabin. The main chamber includes a second chamber positioned in a below portion of the first chamber to overlap the vehicle beltline upon deployment.