seat 10 the seat 10 includes a seat cushion 11 a seat back 12 and a headrest 13 a casing not shown of the airbag system is provided at a side of the seat back 12 in which the airbag 1 is housed in a folded state and covered with a module cover pot shown the airbag 1 is formed such that a sheet such as a cloth and a resin sheet cloth in the embodiment is formed in a bag shape an interior of the airbag 1 is partitioned into a lower chamber 1 a and an upper chamber 1 b with a linear connecting portion a seam 2 in this embodiment the seam 2 connects an occupant side surface 1 p of the airbag 1 and an opposite side surface 1 q together the seam 2 has a substantially l shape having a substantially horizontal section 2 a and a vertical section 2 b as shown in fig2 a the vertical section 2 b of the seam 2 extends vertically and is positioned slightly away from a rear end of the airbag 1 a vertically extending housing chamber 4 is formed at a portion between the vertical section 2 b and the rear rim of the airbag 1 a front end of the horizontal section 2 a of the seam 2 is connected to a front rim of the airbag 1 the housing chamber 4 houses a rod shaped gas generator 3 a longitudinal direction of the gas generator 3 is aligned in a vertical direction the gas generator 3 includes gas jet ports 3 b at.