13 between the two seating frame support tubes 4 are disposed front crossed support tubes 14 which are hinged to each other their upper portions are connected movably to the seat disks 5 and the lower portions are connected to the foot disks 3 at the upper portion of each of the two seating frame support tubes 4 is disposed a seat disk 5 the two seating frame support tubes 4 penetrate the fitting hole 52 of the seat disk 5 at the upper ends of the seating frame support tubes 4 are disposed rotatable joint members 8 via a u shaped hinging member 7 please refer to fig4 and 9 at the same time the joint hole 72 passing through the u shaped hinging member 7 via the joint member is connected to the upper end of the seating frame support tubes 4 and the joint hole 73 of the u shaped hinging member 7 is connected to the fixing hole 83 of the rotatable joint member 8 via another joint member the lower end of each of the seating frame support tubes 4 is connected the sliding sleeve 2 which is sleeved on the side crossed support tubes.