Disclosed is a curtain airbag which has at least one expandable portion formed at a front side thereof; and a tether panel which is coupled to the curtain airbag so as to be disposed on one surface of the curtain airbag, and tightened as the curtain airbag is inflated, so as to allow the at least one expandable portion to protrude toward the other surface that is directed toward the inside of an interior of a vehicle.