To solve a problem that when the collapse of an attitude such as a fall to a front side of an occupant on a rear seat is generated and the occupant strikes a back surface of a seat back of a front seat in emergency such as the head-on collision of the vehicle, the occupant may contact with an operation mechanism or the like of an active head rest through a trim cover.

The trim cover of the seat back 14 is formed in an approximately bag-like shape having an opened lower end. A bag body 18 is provided on a back surface 16-1 of the trim cover, and a hard board 12 is inserted into the bag body and is arranged on the back surface of the seat back. The bag body 18 is provided on an inner surface of the back surface 16-1 of the trim cover, and the hard board 12 is laid between side frames in a state that it can be engaged with the left and right side frames of the seat back frame from the back.