Abstract: In order to promote bloodstream and reduce fatigue, a device of the present invention makes a slight sitting posture change by adding a minute body motion with no necessity of moving a cushion frame or a seat frame though it realizes a supporting pressure change at a wide area, not locally, in a seat cushion or a seat back. It includes a cushioning member (30) for a seat cushion stretched across the cushion frame (10), and a cushioning member (40) for a seat back stretched across the back frame (20). The seat structure of the present invention includes: a lower cloth spring (31a) of a seat cushion or a cloth spring (41) of a seat back stretched across the cushion frame (10) or the back frame (20) via a torsion bar (11); and a cloth spring adjusting member (50) to change a supporting pressure by the cushioning member (30) for the seat cushion and the cushioning member (40) for the seat back through adjusting tension of the lower cloth spring (31a) or the cloth spring (41).