To provide a technique for constituting a vertical device composed by less number of links to dispense with assembly work requiring changes of a position of a large and heavy seat when the vertical device is assembled on the seat.

This seat vertical device is provided with a lower arm held no a member on a floor side, a vertical bracket arranged so as to allow the adjustment of a height for the lower arm, a sector gear arranged between the lower arm and the vertical bracket, and a brake mechanism which is meshed and engaged with the sector gear and includes a pinion. The lower arm and the vertical bracket are mutually connected by a pivotal shaft, and the sector gear and the vertical bracket are mutually connected by a pivotal shaft to engage with the lower arm through a slide member connected with the sector gear so as to slide.