To provide a sitting-straight chair which can be easily and safely assembled and folded, and has high safety.

The sitting-straight chair includes a seat plate 4 for sitting and a bottom plate 2 which is oppositely arranged below the seat plate and set on a floor. The seat plate 4 and the bottom plate 2 are connected via a leg portion 3 so that the height is adjustable. The leg portion 3 includes: a first leg portion 31 including leg columns 310A, 310B which are attached at a prescribed positions on the back side of the seat plate 4 so that one end is turnable; and a second leg portion 32 including leg columns 320A, 320B which are turnably attached at a prescribed position on the surface of the bottom plate 2 where one end is urged by a torsion spring 10, and cross the first leg portion 31 (forming X-shapes), and are pivotally set turnable at the center parts of each other. The seat plate 4 has a plurality of elongated holes 422 where the other end of the first leg portion 31 is locked, and the bottom plate 2 has a support portion by which the other end of the second leg portion 32 is supported.