To provide a support mechanism for a backrest of a chair which can flexibly and accurately follow complicated movement of the back when an upper half body of a seated person is leaned backward.

On the front side of a backrest support arm 2 raised on the back side of a seat plate 1 of a chair, a back rest 3 is pivotally supported by a horizontal shaft m at the lower part of the backrest and at the raised part of the arm 2, and right and left parts of the back side of the backrest 3 are supported higher than the pivotally support part 2a via an elastic or resilient member 4.