To improve a feeling of sitting by arranging a sector gear, which is geared to a pinion gear and journalled rotationally in the vertical direction, and connecting a mounting bracket suspended from the bottom part of a seat to the sector gear.

Mounting brackets 43, 44, which are suspended on the left and right sides of the front and rear parts of a seat frame 4, are connected to a height mechanism respectively. A vertical impact applied to an upper frame 2 is absorbed by means of a shock absorber 35 and a compression coil spring. When an operation handle in the height mechanism is operated so as to be turned, sector gears 51, 61 are turned in the vertical direction via pinion gears 50, 60, so that a seat frame 4 connected to the sector gears 51, 61 is moved vertically, and as a result, the height of a seat cushion (SC) is adjusted.