To improve flexibility of a feed operation of a link in a seat height adjustment device.

The seat height adjustment device 5 capable of adjusting the height with respect to a vehicle floor surface of a vehicle seat 1 includes: a link mechanism 10 which link-connects a seat cushion 3 in a state capable of lifting or lowering with respect to the vehicle floor surface; a drive unit 20 with a brake for transmitting a rotating power to a rear link 12 of the link mechanism 10; and a connection mechanism 30 of the gear provided mediating in a power transmission path between the drive unit 20 with the brake and the link mechanism 10. The connection mechanism 30 of the gear is equipped with a combination of the connecting gear having at least two different kinds of gear ratio (a small-gear pinion gear 31, and small-gear sector gear 33, and large-gear pinion gear 32 and a large-gear sector gear 34), and the gear ratio of power transmission can be changed by switching operation of a switching member 41 provided on the connection mechanism 30 of the gear.