To make it possible for a seat to be lowered without reducing lowering speed when the weight of the seat is light, that is, when no person is seated thereon in the dental treatment chair which enables a seat part to be raised by using a hydraulic cylinder and to be lowered by its weight.

The chair is provided with a cylinder 21 erected on the base part 20, a piston 22 moving up and down through the cylinder 21, and a seat part 10 mounted on the top part of the cylinder, and the seat part 10 is raised by applying pressure on the oil fluid in the cylinder 21, and is lowered by its weight. The chair has a sensor 70 for detecting the lowering speed of the seat part 10 and a drainage passage 40 for discharging an oil liquid from the cylinder 21, and the lowering speed is maintained as predetermined notwithstanding the load weight on the seat 10 by controlling the discharge of the oil fluid from the drainage passage 40 according to the signal by the sensor 70.