To provide a seat sliding device with which an assembly process can be simplified and strength in engagement is improved as well by easily engaging the top end part of a slide rod and a seat board from the upside with one action.

Concerning this seat board sliding device, longitudinal through tube-shaped casings provided on both the sides of the seat board are externally attached to the seat holding parts of both side frames attached to leg parts so as to be slid back and forth, slide rods 7 elastically energized backward are mounted into the seat holding parts of frames, top end parts 8 of the slide rods protruded from the front ends of the seat holding parts are engaged to the seat board, and the seat board can be slid/moved forward against elastic energizing force with the change of a sitting person into reclining attitude and can be slid/moved backward by elastic energizing force with the change of the sitting person into right standing state. Then, engage parts 9 are formed at the top end parts 8 of the slide rods, lock parts 10 are formed on the seat board corresponding to the lock parts 9, engaging instruments 11 are engaged from the direction orthogonal to the slide direction into the engage parts 9, and the engaging instruments 11 are locked to the lock parts 10.