PURPOSE: To improve a riding sensation by improving the assembling property of a seat back main body having a center cushion.

CONSTITUTION: A seat back main body 5 and a center cushion 6 are formed as separate bodies, a seat back part 3 formed by detaining and attaching the center cushion 6 to the seat back main body 5 by a detaining/attaching means is provided, and the detaining/attaching means 7 is constituted of a wire member (wire) 10 provided on the seat back main body 5 and a hook member (hook plate) 13 provided on a cushion frame (wire frame) 12 of the cushion 6. Subsequently, by respectively bending both end parts of the hook member 13, detaining/attaching parts 14a, 14b are formed, and in a state that one end detaining/attaching part 14a of the hook member 13 is attached to the cushion frame 12 so as to be turnable, the center cushion 6 is attached to the seat back main body 5 by hooking the other end detaining/attaching part 14b of the hook member 13 to the wire member 10.