An actuator for a rotary function element, having a housing with at least one pressure means supply and being closed at both sides by a cover, in which housing a piston is guided to reciprocate in a sealing manner, the piston containing diametrically opposed, convolution-like connecting links for a transverse axis of an actuator shaft rotatably mounted in one cover, and having two guide rods firmly anchored in the housing only at one end and engaging into guides in the piston, with the one guide rod anchored in one cover, whereas the other guide rod is anchored in the other cover, and the two guides end blind in the piston in opposite directions.