To provide a support mechanism for a backrest which can softly and accurately follow the complex movement of the back when the bust of a seating person is inclined backward.

In the front of a backrest supporting arm 2 raised behind a seat plate 1 of a chair, a backrest 3 is pivoted by an axis parallel to the rising portion of the arm 2 in the lower part and left and right portions on the back side of the backrest 3 are supported via an elastic member 4 in the upper part from a pivot portion 2a.