A curtain airbag includes: an airbag body of which an upper periphery is fixedly mounted on a body side of the vehicle to be deployable downward to cover an inboard side of the window when an inflation gas flows in; and a connection belt connecting the window and the airbag body, wherein: the connection belt is arranged so as to cover a portion of an end periphery of the airbag body in the front and rear direction and an inflatable portion of the airbag body on an outboard side wall side of the airbag body in a proper arrangement state; and a mark for checking the proper arrangement state of the connection belt is arranged to be visually recognized from at least the outboard side wall side in a vicinity of each of an upper periphery and a lower periphery of the connection belt in the proper arrangement state.