Protective device for closing an opening , in particular a building opening, wherein the protective device comprises: - at least one horizontally movable closing element (26, 30; 126, 130; 226, 230; 326, 330), in particular a high-speed closing element (26, 30; 126, 130; 226, 230; 326, 330), having a closing edge (28, 32; 128, 132; 228, 232; 328, 332), wherein the closing element (26, 30; 126, 130; 226, 230; 326, 330) is horizontally movable in a closing plane between an open position and a closed position, - a horizontal guide on which at least one horizontally movable carriage is arranged, which supports the closing element (26, 30; 126, 130; 226, 230; 326, 330), - a drive for driving the closing element (26, 30; 126, 130; 226, 230; 326, 330), and - a monitoring device having at least one sensor unit for monitoring a closing plane , wherein the sensor unit comprises at least one optical sensor (72, 74; 172, 174; 272, 274; 372, 374), which is horizontally movable together with the closing element (26, 30; 126, 130; 226, 230; 326, 330), wherein the at least one optical sensor (72, 74; 172, 174; 272, 274; 372, 374) is adjacent to the horizontal guide and monitors the closing plane from above, wherein the at least one closing element (26, 30; 126, 130; 226, 230; 326, 330) is held without a guide at its lower end at least in the region of the closing edge (28, 32; 128, 132; 228, 232; 328, 332), and wherein the at least one closing element (26, 30; 126, 130; 226, 230; 326, 330) is configured as a flexible closing element (26, 30; 126, 130; 226, 230; 326, 330). - at least one horizontally movable closing element (26, 30; 126, 130; 226, 230; 326, 330), in particular a high-speed closing element (26, 30; 126, 130; 226, 230; 326, 330), having a closing edge (28, 32; 128, 132; 228, 232; 328, 332), wherein the closing element (26, 30; 126, 130; 226, 230; 326, 330) is horizontally movable in a closing plane between an open position and a closed position, - a horizontal guide on which at least one horizontally movable carriage is arranged, which supports the closing element (26, 30; 126, 130; 226, 230; 326, 330), - a drive for driving the closing element (26, 30; 126, 130; 226, 230; 326, 330), and - a monitoring device having at least one sensor unit for monitoring a closing plane , wherein the sensor unit comprises at least one optical sensor (72, 74; 172, 174; 272, 274; 372, 374), which is horizontally movable together with the closing element (26, 30; 126, 130; 226, 230; 326, 330), wherein the at least one optical sensor (72, 74; 172, 174; 272, 274; 372, 374) is adjacent to the horizontal guide and monitors the closing plane from above, wherein the at least one closing element (26, 30; 126, 130; 226, 230; 326, 330) is held without a guide at its lower end at least in the region of the closing edge (28, 32; 128, 132; 228, 232; 328, 332), and wherein the at least one closing element (26, 30; 126, 130; 226, 230; 326, 330) is configured as a flexible closing element (26, 30; 126, 130; 226, 230; 326, 330). wherein the sensor unit comprises at least one optical sensor (72, 74; 172, 174; 272, 274; 372, 374), which is horizontally movable together with the closing element (26, 30; 126, 130; 226, 230; 326, 330), wherein the at least one optical sensor (72, 74; 172, 174; 272, 274; 372, 374) is adjacent to the horizontal guide and monitors the closing plane from above, wherein the at least one closing element (26, 30; 126, 130; 226, 230; 326, 330) is held without a guide at its lower end at least in the region of the closing edge (28, 32; 128, 132; 228, 232; 328, 332), and wherein the at least one closing element (26, 30; 126, 130; 226, 230; 326, 330) is configured as a flexible closing element (26, 30; 126, 130; 226, 230; 326, 330). <EOS>