An airbag inflating vertically in front of an occupant, wherein the airbag has an intermediate part and upper and lower parts (81, 83), the intermediate part having a relatively small lateral width, and the upper and lower parts (81, 83) having a lateral width (W1, W3) larger than that of the intermediate part during inflation; and the lateral width has a relationship expressed as W2 < W1 < W3 where W1 is the maximum lateral width of the upper part, W2 is the minimum lateral width of the intermediate part, and W3 is the maximum lateral width of the lower part, wherein: a) the airbag has two panels only, namely a front panel and a rear panel, b) said front panel and said rear panel each having sides (1, 2, 3, 11, 12, 13) with: b1) forefront linear sides (1f, 3f, 11f, 13f), b2) fronts (1f', 3f', 11f', 13f') of the sides (1, 3, 11, 13), said fronts being contiguous to the forefront linear sides (1f, 3f, 11f, 13f) and being curved toward the centers of the front and rear panels (80B, 80A), respectively, and b3) rear sides (1r, 3r, 11r, 13r) extending from the curved fronts (1f', 3f', 11f', 13f'), c) said front panel further comprising a side having: c1) a rectangular projection in the center, the projection having sides (15a, 15b) continuing to said side of the front panel, and a side connecting the sides (15a, 15b) of the projection, and c2) sides (14a, 14b) between both sides (15a, 15b) of the projection and the rear sides (11r, 13r), said sides (14a, 14b) being sewn up with the adjacent sides (15a, 15b) of the projection, d) said rear panel further comprising a side extending from the rear sides (1r, 3r) and having: d1) a rectangular cut in the center and d2) sides (4a, 4b) on both sides (51, 53) of the cut, said sides (4a, 4b) on both sides (51, 53) of the cut being sewn together, e) the front panel and the rear panel being sewn together along their peripheries with e1) seams (20, 21) connecting the forefront linear sides (1f, 3f, 11f, 13f) and the curved fronts (1f', 3f', 11f', 13f') of the front panel and the rear panel, wherein, since the curved fronts (1f', 3f', 11f', 13f') are provided, the intermediate part with a small lateral width can be formed, and e2) a seam connecting the rear panel at the rear sides (1r, 3r) extending from the curved fronts (1f', 3f') and the front panel at the side connecting the sides (15a, 15b) of the projection. a) the airbag has two panels only, namely a front panel and a rear panel, b) said front panel and said rear panel each having sides (1, 2, 3, 11, 12, 13) with: b1) forefront linear sides (1f, 3f, 11f, 13f), b2) fronts (1f', 3f', 11f', 13f') of the sides (1, 3, 11, 13), said fronts being contiguous to the forefront linear sides (1f, 3f, 11f, 13f) and being curved toward the centers of the front and rear panels (80B, 80A), respectively, and b3) rear sides (1r, 3r, 11r, 13r) extending from the curved fronts (1f', 3f', 11f', 13f'), b1) forefront linear sides (1f, 3f, 11f, 13f), b2) fronts (1f', 3f', 11f', 13f') of the sides (1, 3, 11, 13), said fronts being contiguous to the forefront linear sides (1f, 3f, 11f, 13f) and being curved toward the centers of the front and rear panels (80B, 80A), respectively, and b3) rear sides (1r, 3r, 11r, 13r) extending from the curved fronts (1f', 3f', 11f', 13f'), c) said front panel further comprising a side having: c1) a rectangular projection in the center, the projection having sides (15a, 15b) continuing to said side of the front panel, and a side connecting the sides (15a, 15b) of the projection, and c2) sides (14a, 14b) between both sides (15a, 15b) of the projection and the rear sides (11r, 13r), said sides (14a, 14b) being sewn up with the adjacent sides (15a, 15b) of the projection, c1) a rectangular projection in the center, the projection having sides (15a, 15b) continuing to said side of the front panel, and a side connecting the sides (15a, 15b) of the projection, and c2) sides (14a, 14b) between both sides (15a, 15b) of the projection and the rear sides (11r, 13r), said sides (14a, 14b) being sewn up with the adjacent sides (15a, 15b) of the projection, d) said rear panel further comprising a side extending from the rear sides (1r, 3r) and having: d1) a rectangular cut in the center and d2) sides (4a, 4b) on both sides (51, 53) of the cut, said sides (4a, 4b) on both sides (51, 53) of the cut being sewn together, d1) a rectangular cut in the center and d2) sides (4a, 4b) on both sides (51, 53) of the cut, said sides (4a, 4b) on both sides (51, 53) of the cut being sewn together, e) the front panel and the rear panel being sewn together along their peripheries with e1) seams (20, 21) connecting the forefront linear sides (1f, 3f, 11f, 13f) and the curved fronts (1f', 3f', 11f', 13f') of the front panel and the rear panel, wherein, since the curved fronts (1f', 3f', 11f', 13f') are provided, the intermediate part with a small lateral width can be formed, and e2) a seam connecting the rear panel at the rear sides (1r, 3r) extending from the curved fronts (1f', 3f') and the front panel at the side connecting the sides (15a, 15b) of the projection. e1) seams (20, 21) connecting the forefront linear sides (1f, 3f, 11f, 13f) and the curved fronts (1f', 3f', 11f', 13f') of the front panel and the rear panel, wherein, since the curved fronts (1f', 3f', 11f', 13f') are provided, the intermediate part with a small lateral width can be formed, and e2) a seam connecting the rear panel at the rear sides (1r, 3r) extending from the curved fronts (1f', 3f') and the front panel at the side connecting the sides (15a, 15b) of the projection.