Abstract: A portable stool has an enclosing wall assembly `A` and a flat seat `B` both made of the same or different plastics. The enclosing wall assembly is rectangular in plan view in its extended state and consists of a pair of right and left side walls (1a,1b) and a pair of front and rear transverse walls (2a,2b). Either the pair of side walls or the pair of transverse walls respectively consist of vertical halves hinged by mediate joints (4) to each other. Each side wall having a front and rear vertical edges is foldably connected by corner joints (5) to the transverse walls at lateral and vertical edges thereof adjacent to the former vertical edges. A back side of the flat seat `B` has at least one members (7) capable of fitting on or in at least one other members (6) formed on an upper end of the enclosing wall assembly `A`, such that the instantly openable and closable stool can withstand the body weight of any user including children, allowing the children to use the stool safely and many times.