Abstract: A rotary cylinder actuator has a piston rod placed in a cylinder, provided in its surface with a guide groove and capable of being moved axially between a position corresponding to a clamping angular position and a position corresponding to an unclamping angular position. A guide member is attached to the cylinder to engage in the guide groove. The piston rod turns from a first angular end position toward a second angular end position for normal turning and turns from the second angular end position toward the first angular end position for reverse turning. The opposite guide surfaces of the guide groove serve as a first guide surface for normal turning and a second guide surface for reverse turning, respectively, so that the guide member moves relative to the piston along different paths in the guide groove while the piston rod turns for normal turning and reverse turning, respectively.