An airbag for a vehicle, the airbag formed as a bag body by sewing together a base cloth, wherein the bag body is configured so that a direction in which the bag body unfolds expands from a base end side of the bag body to a tip end side, and, the bag body includes a tensional force providing mechanism generating a tensional force pulling an upper rim of the bag body toward an outer side by forming a tension line extending so as to cross with the direction in which the bag body unfolds, along a side surface of the bag body.