The finite-sized light beam reflected from a weakly absorbing dielectric slab would experience a longitudinal displacement. The longitudinal displacement of a linear-polarized light beam reflected from the single metal layer is theoretically investigated. It is shown that, in the region of absorbed frequency, the longitudinal displacement of TE polarized light beam is not only positive and very small but also grows slowly. The displacement is saturated to maximum when the incidence angle is very large. The longitudinal displacement of TM polarized light beam is negative and has a peak near 80°. The peak value is about one wavelength. It is also shown that the longitudinal displacement of TE polarized light beam keeps a similar action and its maximum is smaller in the region of reflected frequency. The negative displacement of TM polarized light beam is several wavelengths near 90°.