A work support provided with a highly reliable support function by eliminating negative pressure of an oil chamber communicating with an oil chamber on the outer circumferential side. Since a mechanism (6) for supplying pressurized air to an air passage (41) is provided, a pressure increasing piston member (10) has a transversely split structure, and a plurality of air-passing grooves (54) communicating between the air passage (41) and the pressurized air supply mechanism (6) are made in the lower surface of the first piston member (52) of the pressure increasing piston member (10), only the second piston member (53) of the pressure increasing piston member (10) is driven forcibly with pressurized air when the pressure increasing piston member (10) is moved downward and then the first piston member (52) is moved downward through resilient reset of a sleeve body (9). Consequently, negative pressure in an annular oil chamber (57) communicating with an oil chamber (39) on the outer circumferential side is eliminated and intrusion of air or pressurized air can be prevented.