A lock slider, for a slide fastener, includes an upper wing having an interior surface adapted to be in contact with a pair of fastener tapes, and a wedge-like ridge projecting from the interior surface of the upper wing and extending from a guide post toward a rear end of the upper wing for spreading open the respective closely abutting inner longitudinal edges of the fastener tapes. A tapering fore end portion and a rectangular stem of the ridge has a sharp edge. The upper wing further has, in its interior surface, a recessed portion extending around the fore end portion and the stem for for allowing the inner longitudinal tape edges to be displaced into the recessed portion when they are spread open by the ridge. An aperture for the passage of a locking pawl is formed in the upper wing at an intermediate portion of the ridge. By virtue of the combination of the ridge and the aperture, the abutting inner longitudinal tape edges are sufficiently spread open, and the locking pawl comes into locking engagement with the fastener elements inside the slider without interference with the tape edges.