An object of the invention is to provide a seat rail apparatus, which is capable of locking/unlocking forward/backward sliding of a seat for vehicle in a stepless manner with a simple structure, at an inexpensive cost. A brake disk 51 is attached to the front end of a slide screw 5, which is screwed with a nut 4 disposed within a lower rail 2 and an upper rail 3. A lock mechanism 6 comprises rollers 63 and 64, springs 65 for imparting a force to the rollers in the peripheral direction and a supporting member 61 disposed with the rollers and the springs therein. The springs 65 impart a force to the brake disk 51 and the supporting member 61 to bring the rollers 63 and 64 into a friction engagement; thus the rotation of the slide screw 5 can be locked. Owing to first and second engagement plates 81 and 82, the rollers 63 and 64 are caused to roll against the force imparted by the springs 65, and are released from the friction engagement; thus the slide screw 5 can be unlocked.