To suppress the deviation of the body of a user and to raise the knee linked with back raising by an extremely simple structure.

When a transmission link 14 connected to a retractable frame 5 is inclined and displaced, a linking stopper 33 is displaced clockwise following the transmission link 14 by the energizing force of a spring member 32 and abutted to a second arm 27b in a back/knee linking link 27. When a back raising angle reaches 30°, a roller 30 for linking at the tip of the transmission link 14 reaches the tip of the second arm 27b. When back raising is continued further, the transmission link 14 deviates from the second arm 27b. In this case, since the second arm 27b abuts on the linking stopper 33, the back/knee linking link 27 maintains the abutting state and a knee raising angle is kept at about 20°.