A seat suspension mechanism comprising: an upper frame and a lower frame which are supported so as to be capable of a separating and approaching operation relative to each other, with a link mechanism therebetween; and a spring mechanism which elastically biases the upper frame, wherein the spring mechanism includes: a first spring part which biases the upper frame in a direction in which the upper frame separates from the lower frame; and a second spring part which, when the upper frame is in a predetermined up-down movement range including a balanced point, exhibits a spring characteristic of biasing the upper frame in a direction in which the upper frame approaches the lower frame, to create a constant load spring characteristic by being combined with the first spring part, the seat suspension mechanism further comprising a body weight adjustment mechanism which adjusts a position of the upper frame to the balanced point in a state in which a person is supported, wherein the first spring part includes a lower frame-side torsion bar which is connected to the lower frame and is twisted by movement of the link mechanism and an upper frame-side torsion bar (141a, b) which is connected to the upper frame and is twisted by the movement of the link mechanism, wherein the body weight adjustment mechanism includes: an elastic force adjusting unit which is connected to the upper frame-side torsion bar (141a, b) and adjusts the position of the upper frame by twisting the upper frame-side torsion bar (141a, b); a torsion angle detection sensor which detects a torsion angle of the lower frame-side torsion bar; and a control unit which compares the torsion angle of the lower frame-side torsion bar detected by the torsion angle detection sensor with a balanced point torsion angle that the lower frame-side torsion bar has when the upper frame is at the balanced point, and outputs, to the elastic force adjusting unit, a control signal instructing to twist the upper frame-side torsion bar (141a, b) by a predetermined amount to make the torsion angle of the lower frame-side torsion bar agree with the balanced point torsion angle. a first spring part which biases the upper frame in a direction in which the upper frame separates from the lower frame; and a second spring part which, when the upper frame is in a predetermined up-down movement range including a balanced point, exhibits a spring characteristic of biasing the upper frame in a direction in which the upper frame approaches the lower frame, to create a constant load spring characteristic by being combined with the first spring part, the seat suspension mechanism further comprising a body weight adjustment mechanism which adjusts a position of the upper frame to the balanced point in a state in which a person is supported, wherein the first spring part includes a lower frame-side torsion bar which is connected to the lower frame and is twisted by movement of the link mechanism and an upper frame-side torsion bar (141a, b) which is connected to the upper frame and is twisted by the movement of the link mechanism, wherein the body weight adjustment mechanism includes: an elastic force adjusting unit which is connected to the upper frame-side torsion bar (141a, b) and adjusts the position of the upper frame by twisting the upper frame-side torsion bar (141a, b); a torsion angle detection sensor which detects a torsion angle of the lower frame-side torsion bar; and a control unit which compares the torsion angle of the lower frame-side torsion bar detected by the torsion angle detection sensor with a balanced point torsion angle that the lower frame-side torsion bar has when the upper frame is at the balanced point, and outputs, to the elastic force adjusting unit, a control signal instructing to twist the upper frame-side torsion bar (141a, b) by a predetermined amount to make the torsion angle of the lower frame-side torsion bar agree with the balanced point torsion angle. an elastic force adjusting unit which is connected to the upper frame-side torsion bar (141a, b) and adjusts the position of the upper frame by twisting the upper frame-side torsion bar (141a, b); a torsion angle detection sensor which detects a torsion angle of the lower frame-side torsion bar; and a control unit which compares the torsion angle of the lower frame-side torsion bar detected by the torsion angle detection sensor with a balanced point torsion angle that the lower frame-side torsion bar has when the upper frame is at the balanced point, and outputs, to the elastic force adjusting unit, a control signal instructing to twist the upper frame-side torsion bar (141a, b) by a predetermined amount to make the torsion angle of the lower frame-side torsion bar agree with the balanced point torsion angle.