A seat slide adjuster for adjusting a position of a seat in a front-rear direction, comprising: a lower rail; an upper rail that is slidably provided to the lower rail and connected to a seat frame that comprises a seat; and a locking mechanism that locks the upper rail to the lower rail at an appropriate slide position, wherein the lower rail comprises locking tooth parts on both sides in a longitudinal direction that protrude downward in a comb shape, and wherein the locking mechanism comprises: a locking plate that corresponds to the locking tooth parts located on both sides of the lower rail in the longitudinal direction that includes an engaged portion that engages with the locking tooth parts formed in two rows; and a locking mechanism support member supported on the upper rail side and including elastic support means for elastically supporting the locking plate so as to allow the engaged portion of the locking plate to engage with the locking tooth parts, characterized in that the locking mechanism comprises a wedge structure including wedge portions that are disposed at a position adjacent to each of the longitudinal edge portions along the longitudinal direction of the upper rail in the locking plate and elastically supported by another elastic support means provided to the locking mechanism support member, and make contact with each of the longitudinal edge portions of the locking plate to press the locking plate inward during locking. a lower rail; an upper rail that is slidably provided to the lower rail and connected to a seat frame that comprises a seat; and a locking mechanism that locks the upper rail to the lower rail at an appropriate slide position, wherein the lower rail comprises locking tooth parts on both sides in a longitudinal direction that protrude downward in a comb shape, and wherein the locking mechanism comprises: a locking plate that corresponds to the locking tooth parts located on both sides of the lower rail in the longitudinal direction that includes an engaged portion that engages with the locking tooth parts formed in two rows; and a locking mechanism support member supported on the upper rail side and including elastic support means for elastically supporting the locking plate so as to allow the engaged portion of the locking plate to engage with the locking tooth parts, characterized in that a locking plate that corresponds to the locking tooth parts located on both sides of the lower rail in the longitudinal direction that includes an engaged portion that engages with the locking tooth parts formed in two rows; and a locking mechanism support member supported on the upper rail side and including elastic support means for elastically supporting the locking plate so as to allow the engaged portion of the locking plate to engage with the locking tooth parts, characterized in that the locking mechanism comprises a wedge structure including wedge portions that are disposed at a position adjacent to each of the longitudinal edge portions along the longitudinal direction of the upper rail in the locking plate and elastically supported by another elastic support means provided to the locking mechanism support member, and make contact with each of the longitudinal edge portions of the locking plate to press the locking plate inward during locking.