A curtain airbag capable for preventing occupant ejection from a vehicle, particularly in the vicinity of a rearmost seat, while reducing the output required of an inflator associated with the airbag. The curtain airbag is stored at an upper position on a side surface portion of the occupant compartment of the vehicle, so as to be inflated and deployed along the side surface portion. The curtain airbag includes an inflation region and a wide portion in the form of non-inflatable cloth. The wide portion is attached to a rear edge portion of the inflation region and also attached to at least one point of the vehicle rearward of a front-side pillar, the front-side pillar being located forward of rearmost side window. The wide portion has a shape that allows it to be strained, by the inflated and deployed inflation region, into a generally planar configuration over an assumed strike area of the rearmost side window.