A side-impact airbag is disposed in a space between a side wall portion of a vehicle and a seat. The side-impact airbag includes a plurality of chambers partitioned in the airbag, a communication passageway formed in a front region of the airbag in a front-and-back direction of the vehicle, and a tether disposed between two of the plurality of chambers located at a rear region of the airbag in the front-and-back direction. The communication passageway allows at least two of the plurality of chambers to communicate with each other at the front region. An inflator supplies the gas to expand the side-impact airbag. The gas from the inflator is lead toward the communication passage through one of the partitioned chambers.