PURPOSE: To improve the durability of a spring member, and simultaneously, to obtain smooth flexible repulsive action to a seat back by interveningly providing a spring member to elastically energize a back supporting lever to the front side between the edge of a seat side bracket and the edge of a back side bracket provided on the back supporting lever.

CONSTITUTION: To a leg post 2 projectingly provided from a leg blade 1 to an upper side, a seat receiving member 3 is connected, L-shaped fittings 7 are fitted to both edges of a bracket 4 rigidly mounted to a top board 3a, respectively, and a pin 8 is projectingly provided from the outer side surface of a bent part 7a towards an external side. The upper edge of a back supporting lever 9 to be turnably pivot-supported to the seat receiving member 3 is extended, L-shaped fittings 12 are fitted to both edges of a bracket 10 for locking a seat back 11, and a pin 13 is projectingly provided from the outer side surface towards the external side. With the pins 8 and 13, both edges of a spring member 14, in which coil springs are elastically provided turnably, respectively, are engaged. Even when the member 14 is made somewhat large, the fitting can be executed without inconvenience, the degree of a fatigue due to a repeated utilization is made small, and thereby, the durability can be improved.