To give good appearance, make a seat comfortable to sit on, and make it more difficult for force in the direction of torsion to act on a seat back.

A knuckle structure for a seat comprises a pair of widthwise brackets 3 fixed to both sides of the rear part of a seat bottom, a connection rod 4 extending between the brackets 3 so as to be freely turnable around its axis, a pair of widthwise knuckle plates 5 fixed to both sides of the lower part of a seat back so as to correspond to the brackets 3 and supported by pivots so as to be freely turnable around the connection rod 4, a spiral spring 6 for urging the seat back so as to incline it forward, and a lock mechanism 7 for locking the seat back into an arbitrary position against the urging force of the spiral spring 6 by operating an operation lever 8 and for releasing the lock. The spiral spring 6 is provided on the opposite sides of the pair of knuckle plates 5.