To make a waist part supportable in accordance with the difference in physiques by a simple constitution.

The seat 10 in the reverse surface side of each cushion material, is provided with a belt member 200 locking respectively one end to a suitable part in a frame 30 for a back part the other end to a suitable part in a frame 40 for a seat part to be arranged along the direction of vertical section in the vicinity of at least one side part of either the right or left of a seated person to additionally set a length in a range mutually overlapped with the cushion material shorter than a length in an extended condition of the cushion material in this range to be further formed of material of rigidity higher than that of each of the cushion material. Since this belt member 200 is arranged, by relatively protruding a location corresponding to the belt member when a person is seated, supporting of a waist part of the seated person can be attained. In addition, a relatively protruded position of the location corresponding to the belt member is different by physique (the weight of body or the like) of the seated person, so that a waist part support function in accordance with a difference between physiques can be displayed, seating comfort can be improved.