A stretching structure in which upholstery material can be installed in a frame body, enough deflection allowance of the upholstery material can be ensured. Upholstery material (14) is installed on the surface of a frame body (13) in a closed-loop shape, and is used as the seat or the backrest of a chair. A cross-sectional shape perpendicular to the longitudinal direction of each of the sections of the frame body (13) is provided with an upholstery material receiving section (21) having an engaging groove (20) which is open to the back surface side, and a U-shape section (23) having a recessed groove (22) which is open to the surface side inward more than the engaging groove (20). The upholstery material (14) is wound from the surface of the frame body (13) to the back surface side along the outer peripheral surface, and the edge section is fitted in the engaging groove (20) and tied up, thereby installing the upholstery material (14) in such a manner as to cover the surface of the U-shape section (23).