Abstract: A device for monitoring the relative position of synchronously moved valve cases in safety valves controlling pneumatic or hydraulic drives. Fixedly attached to each valve case and symmetrically disposed one to the other is a member of a metallic material, arranged proximate to the free ends of a pivotally mounted magnetic bar. The axis of rotation of the magnetic bar, which is aligned with the synchronous position of the valve cases in its basic position, is parallel to the direction of movement of the valve cases, and a swing of the magnetic bar, which only occurs in the event of a defect, i.e. if the valve cases do not operate in synchronism, provides a signal for making the monitored drive inoperative.