A closure device is provided for a cabriolet roof structure having at least one closure element which can assume an open position and a closed position, wherein the closure device is at least at times assigned a sensor device which can detect the open position and the closed position. According to the invention, it is provided that the sensor device comprises only one sensor which generates a first signal when the open position is reached and which generates a second signal when the closed position is reached. The invention also relates to a roof structure for a cabriolet and to a method for monitoring a closure device for a cabriolet roof structure.