To provide a small stool with a warp plate for straight sitting, which prevents adverse effects on human bones even when keeping the straight-sitting posture with legs folded underneath for a long time.

A warp-plate seat part onto which a human sacrum of buttocks naturally fits is cut off in the center and reconnected with a hinge, a leather material or the like. Metal fittings for mounting leg parts are attached on a back side of the warp-plate seat part. Further, the stool includes: a leg reinforcement member for preventing an X-shaped collapsible legs from closing when opening them in the X-shape; and a foot-stabilizing cushioning material attached to a floor-contact face of the X-shaped collapsible legs.