An airbag is disposed over almost the entire length along a roof side rail. The airbag comprises a duct formed with a gas introduction hole and a plurality of chambers communicating with the duct. The duct comprises a long first duct part and a short second duct part arranged with the gas introduction hole being provided therebetween. The inlet of a chamber closest to the gas introduction hole in the first duct part has a shape that becomes narrower gradually from the upper part toward the lower part. The passage width of the uppermost part of the inlet of the closest chamber is greater than the depth to the narrowest part thereof, and the passage width of the narrowest part is narrower than the passage width of the first duct part around the inlet thereof.