An airbag for a vehicle includes an airbag cushion and a tether. The airbag cushion is installed on one side of a seat and is composed of multiple chambers. When the airbag is deployed, the airbag cushion extends to a front side of the seat and the chambers are consecutively arranged in a superior-inferior direction of the seat. The tether connects the chambers to each other or connects one chamber to a vehicle body. The multiple chambers include a normal chamber deployed to the front side of the seat and a confining chamber bent toward an occupant when being deployed. The confining chamber is pressed by the adjacent normal chamber, or pulled or pressed by the tether. Thus, a front half portion of the confining chamber is folded toward a rear half portion of the confining chamber, thereby coming into tight contact with the occupant.