In a seat sliding device having a lock mechanism disposed within an inner space defined by an upper rail and a lower rail, to tilt a lock lever from the inner space toward the outside of the rails, a torsion bar 40 is used as urging means for urging the lock lever 31 in the lock mechanism 30. When an operation lever 50 is in its inoperative state, the lock lever is urged by torsional force of the torsion bar in its locking direction, claws 32 are fitted into engaging holes 11a and 21a, from the inside of the lower rail 10 and upper rail 20 toward the outside of the rails, to be placed in its locking position. When the operation lever is operated, the lock lever is tilted against the torsional force of the torsion bar, the fitted state with the claws being fitted into the engaging holes of both rails is released, to make the upper rail movable relative to the lower rail.