PURPOSE: To allow a person to be seated for many hours by dividing a back rest into the upper half part and the lower half part of the back rest, and displacing the lower half part of the back rest to the front side of the seat surface, while inclining backward, when the back rest surface is inclined backward.

CONSTITUTION: The back rest lower half part 7 attaches integrally the upper end 7a to the upper end part 6a of a supporting frame 6, and on the other hand, attaches the lower end 7b to the rear end of the seat surface 2 and forms them integrally. In this case, the front side of the back rest lower half part 7 becomes a circular arc-like side face shape whose curvature is small in the vicinity of a backward inclination terminal, although it is circular arc-like side face shape whose curvature is comparatively large before it is inclined backward. This face means a fact that, while the upper and the lower half parts 3, 7 of the back rest are inclining backward integrally, only the back rest lower half part 7 is displaced so as to advance in the forward direction. When the back rest is inclined backward, the upper half part 3 executes an operation for following such a posture variation as the upper half part of the body of a seated person is inclined backward, and on the other hand, the lower half part 7 can follow a posture variation of the lumber vertebra B and the upper part of the pelvis of the seated person, therefore, an excellent seated sensation is obtained.