To obtain a comfortable sitting attitudes by securing a broader space about feet even under the reclining state while preventing increase in the weight of a seat back frame, a link or the like along with a reduction in cost and a lighter weight.

A seat back 12 is divided in two vertically and the upper half 14a of the seat back frame is supported on a body panel 20 through link arms 24 and 25 to receive a load working on the seat back 12 on the side of the body panel 20. A rotating force of a motor 30 is converted to a linear motion with a feed screw mechanism so that the link arms 24 and 25 are operated to turn to shift the seat back 12 to the reclining position and the mid-folded position while a seat cushion frame 13 linked to the lower half part 14b of the seat back frame 13 is supported through a link arm 22 at the front end thereof thereby interlocking the cushion tilt of the seat cushion 11.