The piston 10 is constituted so as to rotate relative to the first cylinder 8 while moving relative to the first cylinder 8 in the axial direction when fluid is supplied to and discharged from the interior of the first cylinder 8, and the piston 16 is constituted to be incapable of rotation relative to the second cylinder 9, but capable of movement relative to the second cylinder 9 in the axial direction. Hence when fluid is supplied to and discharged from the interior of the first cylinder 8, the piston 10 is caused to rotate relative to the first cylinder 8 while moving relative to the first cylinder 8 in the axial direction, and the piston 16 is caused to move relative to the second cylinder 9 in the axial direction but prevented from rotating relative to the second cylinder 9. As a result, torsional elasticity is generated in shaft portions 4a and 5a.