A piston for a setting device operated by fluid power is suggested. The piston includes a support body having an annular recess with an axis coinciding with the longitudinal axis of the piston, such recess having at least one annular permanent magnet arrangement therein. The permanent magnet arrangement is segmented in its peripheral direction and consists of at least two accurate magnet segments. The annular recess is in the form of an annular groove with a radially outwardly directed groove opening into which the magnet segments are inserted radially from the outside with the formation of an annular configuration.