A lens unit for a scanning device includes a frame having a hollow cylindrical shape, the frame being defined with a lens contact portion therein, a lens accommodated in the frame contacting the lens contact portion, and a retainer accommodated in the frame to retain the lens in position, the retainer having a hollow cylindrical shape, one end side face of the retainer contacting a peripheral portion of the lens received by the frame, an other end portion of the retainer being secured to the frame so that the retainer presses the lens toward the lens contact portion of the frame to fix the lens to the frame. Deformation of the frame, lens and retainer due to the load generated as the retainer presses the lens absorbs deformation of the frame, lens and retainer due to temperature change at least within a predetermined temperature range.