To set webbing so that an inflation pressure of an air bag can instantaneously cut apart a tear portion of a trim cover without causing a tear of the trim cover.

The webbing strips 14 and 15 are separate from the trim cover 11 except for one terminal stitched up with the tear portion 11a of the trim cover 11. One webbing strip 14 is wrapped behind a side plate 13b from a front portion of a back pad 12, and the other webbing strip 15 is wrapped behind the side plate 13b from a front portion of an air bag module 10. Separately from terminals of the trim cover 11, the other terminal of each webbing strip 14, 15 is tensely secured to the side plate 13b of a seat back frame 13a.