A lumbar support device includes a seat-back having a first upstanding frame and a second upstanding frame spaced therefrom in the lateral direction, a bilateral asymmetrical torsion spring having a first end portion which is rotatably supported by the first upstanding frame and a second end portion, a supporting plate mounted on the bilateral asymmetrical torsion spring, and a driving device disposed between the second upstanding frame and the second end portion of the bilateral asymmetrical torsion spring for moving the bilateral asymmetrical torsion spring in the direction of the thickness of the seat-back, wherein a degree of a deformation of the first end portion of the bilateral asymmetrical torsion spring is less than that of the second end portion of the bilateral asymmetrical torsion spring.