To provide a curtain airbag which enables a first impact protection performance upon a side collision to be consistent with an ejection prevention performance on a rollover.

The curtain airbag includes: a main cushion 130 which includes a series of chambers 142, 144 inflatable along the side surface of a cabin, each chamber being inflated to both vehicle interior and exterior sides on the basis of a common plane 160; and sub cushions 172, 174 which are arranged at the vehicle exterior side of the main cushion and whose upper and lower ends extend over the upper and lower portions of a door belt line 180 of a vehicle. The sub cushions 172, 174 are overlapped or come in contact with an assumed impact areas 192, 194 of the main cushion 130 where a collision of an occupant is assumed in the direction of viewing the outside from the inside of the cabin, and whose interior spaces are communicated with the main cushion through vent holes 200, 202.