To easily secure installation rigidity, enable separate adjustment of rotation torque of the seat, and reduce manufacturing cost.

The rotation support mechanism for a seat of a chair is provided with a support part 3 diametrically projected from an outer circumferential surface 1a of a leg 1 for a seat side installation part 2 to be placed, a presser part 4 to rotatably hold the seat side installation part 2 to the support part, a first slide member 5 provided between the support part 3 and the seat side installation part 2, and a second slide member 6 provided between the seat side installation part 2 and the presser part 4. In addition, an adjusting mechanism 7 is provided projecting from the presser part 4 toward the second slide member 6 or from the support part 3 toward the first slide member 5 to adjust friction force generated between the second slide member 6 or the first slide member 5 and the seat side installation member 2 by adjusting its projection quantity.

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