A seat slide device is switchable between a locked state and an unlocked state. When the seat slide device in the locked state, a lock member attached to an upper rail is engaged with a lock hole formed in a lower rail. This restricts movement of the upper rail with respect to the lower rail. When the seat slide device is in the unlocked state, the lock member is separated from the lock hole to permit the upper rail to move with respect to the lower rail. In the unlocked state, a gap is formed between a movable portion and the lower rail. In contrast, in the locked state, the movable portion contacts the lower rail and the gap between the movable portion and the lower rail is eliminated.