PURPOSE: To provide a rocking mechanism for a backrest supporting the longitudinal rocking of the backrest and capable of smoothly realizing the rocking action with a simple and compact structure.

CONSTITUTION: A base block 2 integrally formed with a seat plate 3 on the upper side is nearly horizontally provided at the upper end section of the leg stanchion 1 of a chair, the lower end side of a backrest support member 4 fitted with a backrest is extended to the lower side of the seat plate 3 of the base block 2, the middle of the extension section 41 of the support member 4 is pivotally fitted on the rear end side of the base block 2 through a horizontal shaft A, and a horizontal shaft C is provided at the front end section of the extension section 41 through an oblong hole B formed at the middle section of the base block 2. The front side 5a of a plate spring 5 opened on the rear side 5b as a free end is fitted in the front of the base block 2, the shaft C is brought into contact with the rear side 5b of the plate spring 5, and the backward inclination of the backrest is elastically supported by the repulsion of the plate spring 5.