Recently, powered exoskeletons for strengthening have been used in heavy work like construction industry. However, most of the powered exoskeletons can only strengthen a part of the body of a user. A small and light actuator which generates great force is required for the powered exoskeletons strengthening the whole body. It is expected that a thin hydraulic artificial muscle is such a high efficient actuator. In this paper, we developed the thin hydraulic artificial muscle that has a mass of 22 g/m and a diameter of 6 mm. It has 1262 N generation force in applying 5 MPa. We also developed and evaluated a powered exoskeleton for strengthening hands using the thin hydraulic artificial muscle. The result showed that the artificial muscle strengthened the grip force of a finger about 300 N under a pressure of 3 MPa. Therefore the thin hydraulic artificial muscle is effective for strengthening powered exoskeletons.