Abstract: A locking device for a rotatable chair or table includes a middle vertical tube, an annular closing member fixed on an upper end of the middle vertical tube, a protective outer tube fitting around the middle vertical tube, having an inner wall provided with the same shape of as the annular closing member and having a plurality of screw holes formed in an upper end portion, and screws screwing in the screw holes of the protective outer tube. The screws can be screwed inward tightly against an outer surface of a piston rod extending out of the air pressure bar contained in the middle vertical rod. When the piston rod is locked immovable by the screws, the chair or table is also locked immovable, being fixed tightly with an upper end of the piston rod.