Protective device (10, 110, 210, 310) for closing an opening (14, 114), in particular a building opening, wherein the protective device (10, 110, 210, 310) 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 (18, 118, 218, 318) between an open position and a closed position, - a horizontal guide (48, 148, 248, 348) on which at least one horizontally movable carriage (58, 60, 158, 160) is arranged, which supports the closing element (26, 30; 126, 130; 226, 230; 326, 330), - a drive (38, 238) for driving the closing element (26, 30; 126, 130; 226, 230; 326, 330), and - a monitoring device (68, 168, 268, 368) having at least one sensor unit (70, 170, 270, 370) for monitoring a closing plane (18, 118, 218, 318), wherein the sensor unit (70, 170, 270, 370) 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 (48, 148, 248, 348) and monitors the closing plane (18, 118, 218, 318) 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 (50, 52, 150, 152) 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 (18, 118, 218, 318) between an open position and a closed position, - a horizontal guide (48, 148, 248, 348) on which at least one horizontally movable carriage (58, 60, 158, 160) is arranged, which supports the closing element (26, 30; 126, 130; 226, 230; 326, 330), - a drive (38, 238) for driving the closing element (26, 30; 126, 130; 226, 230; 326, 330), and - a monitoring device (68, 168, 268, 368) having at least one sensor unit (70, 170, 270, 370) for monitoring a closing plane (18, 118, 218, 318), wherein the sensor unit (70, 170, 270, 370) 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 (48, 148, 248, 348) and monitors the closing plane (18, 118, 218, 318) 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 (50, 52, 150, 152) 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 (70, 170, 270, 370) 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 (48, 148, 248, 348) and monitors the closing plane (18, 118, 218, 318) 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 (50, 52, 150, 152) 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).