A seat structure includes a torsion bar 12 disposed along the width direction, and an arm 13 which is connected to the torsion bar 12, enforced in the direction of falling backward at a normal state by the torsion bar 12 in the rear of a seat cushion 10, and disposed is a flat-type supporting member 15 between the supporting frame 14 supported by the arm 13 and a frame member provided in front of the seat cushion, being strained by the elastic force of the torsion bar 12. Through this structure, the spring characteristic to give an influence on comfortability and riding comfort at the time of being seated can be given by the torsion bar 12, so that the number of coil springs which have been necessary in the vicinity of the body side can be reduced or the use of the coil spring can be stopped.