To facilitate folding-up operation by adjustably arranging a connecting position of a lower link lever of a lower link frame, and arranging a seat back frame so that a fixed condition or an unfixed condition with a front leg frame can be selected.

Lower parts of crossing parts of front and rear leg frames 1 and 2 are connected to each other by a lower link frame 4, and in the lower link frame 4, a base part of its lower link lever 41 is pivotally joined to a side lever part 22 of the rear leg frame 2, and a front lever part 42 is hookingly fastened to a hookingly fastening receiving part 13 so as to be selectable/ adjustable. At folding-up and housing time, a hooking lever 54 of a seat back frame 5 is separated from a receiving shaft, and the connection of the seat back frame 5 and the front leg frame 1 is released, and the seat back frame 5 can be independently rotated with a pivotally joining part with an upper link frame 3 as the center. Therefore, the upper link frame 3 is pushed downward, and when the seat back frame 5 is bent forward, it can be easily folded up.