To improve the operability of covering by simplifying the covering of a seat back trim cover, to improve appearance by reducing costs for treating the backside of a seat back and to facilitate the assembling of a side air bag unit.

A vehicle seat is provided with a covering port brought close to the bottom side of a seat back 12 for one side and opened in the backside 13 of the seat back 12, a slit 22 extended from the top corner side of the covering port positioned outside according to seat installing to the top 14 side of the backside 13 thereof, a seat back trim cover 17 having a fastener 24 connected to the slit 22, and a seat back trim board 26 assembled to a seat back frame from the upper side of the trim cover 17 by covering the covering port of the trim cover 17 in the backside 13 of the seat back 12.