PURPOSE: To detect the state of a fishing net visually by mounting a light emission body or ultrasonic wave oscillator on the fishing net and mounting a photoelectric converting device or ultrasonic wave receiver and an electric signal transmitting device on a rope drawing the fishing net.

CONSTITUTION: For example, light emission bodies 7 are mounted at the entrance part 6a, intermediate part 6b, and terminal part 6c of the fishing net 6. Then the photoelectric converting device 8 such as a video camera mounted on the rope 5 drawing the fishing net 6 detects and converts light from the light emission bodies 7 into a voltage signal. This voltage signal is transmitted by the transmitting device 9 provided integrally with or separately from the photoelectric converting device 8. This sent signal is received by a receiving device provided on a ship 4 through a cord embedded in the rope 5 and displayed on a display device such as a CRT. Consequently, the state of the fishing net 6 is detected at all times.