To remove shocks caused by rapid drive changes occurring when a hydraulic drive motor to ascend or descend a dental treatment chair or tilt its back rest starts or ends its operation.

Revolutions of a hydraulic pump driving motor 9 to drive a cylinder 7 for ascending or descending a dental treatment chair and a cylinder 8 for tilting its backrest 4 are in the control of the set values of acceleration time or deceleration time input to an inverter 11 beforehand, so that when the motor 9 starts, it may gradually accelerate from a low speed and at the end of its operation, its revolutions may gradually slow down to avoid shocks caused by rapid changes in the motor drive conditions at the time of its operation start or end.