Device for monitoring a closure of a locking device, in particular for doors or windows

An adapter rail (7) on door or window casement is coupled to a fixer bolt (2) and slides with this and a sensor (13) whose c transducer (15) and receiver (18) are fitted to rail and fixer frame. An evaluator (29)checks that both transducer and receiver in their mutual bolting bolting position so the bolt (2) engages or whether again the transducer (15) and receiver (18) are in a release position to allow the bolt to disengage. The transducer (15) should be on the rail (7) and receiver on fixed frame and t evaluator is connected to both transducer and receiver as linked by mechanical or non-contact signal transmission. The rail (7) fixed to the casement via block (8) and has a hole (12) for positive bolt securement.