PURPOSE: To facilitate precision control for parts of a seat back frame, achieve light weight, and reduce the number of the party items.

CONSTITUTION: A cushion frame 52 is connected to the lower side of a seat back frame 22. A bracket 30 having a long hole 38 is diagonally fixed on the seat back frame 22. A pin 40 of a body bracket 24 is penetrated into the long hole 38, so the body bracket 24 is fixed on the side of a body 12. An end part 28A of a connection rod 28 is rotatably inserted into the seat back frame 22, and a connection plate 48 is caulked with a vertical part 28B of the connection rod 28. The connection plate 48 is rotatably connected to the body bracket 24 through a link 26. A middle part 70 of a seat back seating surface 72 is connected to a horizontal part 28C of the connection rod 28.