Abstract: A fluid-operated actuator comprising a chamber part (1) with at least one chamber (12, 13), a carrier (3) which is rotatably mounted in the chamber part (1), and a plunger piston (2) in the form of a circular arc. The plunger piston can be moved along the circular arc and thereby rotate about the carrier's axis of rotation (50) since it extends in the chamber and is arranged to connect with the carrier (3). According to the invention the plunger piston (2) via a sealing device (32, 33, 42-45) is passed sealingly through an opening (14, 15) in the chamber (12, 13), there being a clearance between the surface of that section of the plunger piston which is located in the chamber and the chamber walls. Furthermore that section of the plunger piston which is located outside the chamber (12, 13) is supported along its radially outer surface by a support part (4) which is attached to the chamber part (1).