Abstract: A mechanical lifting device, preferably for a chair seat, comprising two reciprocally telescoping parts, one of which is attached to a chair support, whereas the other part is attached to the chair seat, both parts being mutually spring loaded, a locking screw being pivoted to the first part inside one of its ends and revolving about the longitudinal axis of the part, that the second part comprises an outer locking tube coaxially surrounding a hollow pressure rod and a number of locking nuts, e.g. three, intended for engagement with the locking screw and provided inside the locking tube, the locking nuts being mutually spring loaded and when the pressure rod is actuated enabling the locking screw to turn relative to the locking nuts when they move in the longitudinal direction of the lifting device, thereby the mutual position of the two parts is also changed. When actuation of the pressure rod stops a locking engagement is achieved between locking nuts and locking screw by the aid of the mutual spring load.