A seat sliding apparatus for vehicles is provided. The seat sliding apparatus includes a track and a guide bracket with legs connected to stepped locking pins to regulate the locked state of the locking pins which engage the track. In one embodiment, the track includes through-holes which are engaged by the locking pins. Accordingly to another aspect of the apparatus, a plastic bushing is provided to prevent horizontal/vertical floating of the locking pins, thereby minimizing the play of the locking pins and prevent floating. The apparatus also may simultaneously prevent a half-locked state of the locking pins.