The present invention provides a seat-sliding apparatus for a vehicle. The seat-sliding apparatus includes a lower rail fixed to the vehicle, an upper rail, which is coupled to a seat and slides along the lower rail, a rail groove portion, which is formed in at least one surface of the lower rail and has therein a plurality of grooves, a tooth portion having teeth positioned between the grooves in the rail groove portion, a sloped portion, which is positioned at at least a portion of the rail groove portion and in which heights of the teeth of the tooth portion decrease toward a center of the rail groove portion from one end of the rail groove portion in a longitudinal direction, and a locking unit, which is positioned at the upper rail and is selectively engaged with the rail groove portion so as to hold the upper rail.