A powered seat slide device comprises upper and lower guide rails slidably fitted to each other, and a rail drive mechanism with a reversible motor and a reduction gear box. The drive mechanism includes a screw-threaded shaft being rotatable about its axis through the motor and the gear box and installed on one of the upper and lower guide rails, and a nut member being installed on another rail and screwed into the screw-threaded shaft for a relative axial motion of said screw-threaded shaft with respect to said nut member. An elastic shock-absorbing damper assembly (including essentially annular elastomeric damper) is supported on the screw-threaded shaft in front and in rear of the nut member. The damper assembly includes at least a plastic washer interposed between the nut member and the damper. The plastic washer has a radially and inwardly tapered and circumferentially curved face to define a substantially frusto-conical hollow facing to the flat side wall of the damper, and thus provide an elastic-deformation permissible space of the damper.