A machine-tool power chuck operated by pressurized gas supplied from an gas pressurizing source has a piston-cylinder actuator having a piston dividing the piston chamber into a rear pushing side and a front pulling side. When the pressurized gas is supplied into the pushing side, a wedge plunger in a sliding chamber is caused to shift whereby workpiece chucking jaws are opened, and, when the pressurized gas is supplied into the pulling side, the jaws are moved inward for chucking the workpiece. Oil mist supply passages communicate at the opposite ends thereof respectively to the pushing and pulling sides of the actuator and have, at a branching junction thereof from which a gas injection passage branches off, a control valve in the form of a shuttle valve having a shiftable ball. The control valve operates to cut off communication of the gas injection passage with one of the pushing and pulling sides, to which the pressurized gas is not supplied. The pressurized gas is provided with an oil mist for lubrication. When the piston is operated, the pressurized gas acting on the piston and containing the oil mist is allowed to flow via the control valve into the gas injection passage and then into the sliding chamber for the wedge plunger, thereby lubricating the rubbing surfaces of the sliding chamber and the wedge plunger as well as other adjoining members.