To eliminate a situation of necessarily giving direction while watching the present-boat and a towboat in drawing a net using a GPS receiver.

In a parent boat, an antenna 2 is installed on the GPS receiver 1, the GPS receiver 1 is connected to a control device 3 comprising a CPU, a signal converting function 3a, a signal processing function 3b, and an image memory function 3c are installed on the control device 3, a radio receiver 4 provided with an antenna 5 is connected to the control device 3, and a display 6 is connected to an image memory function 3c of the control device 3. On the towboat, an antenna 8 is installed on a GPS receiver 7, the GPS receiver 8 is connected to the control device 9 comprising a CPU, and on the control device 9, a signal converting function 9a, a signal processing function 9b, and an image memory function 9c are installed. A radio transmitter 10 provided with an antenna 11 is connected to the control device 9, and a display device 12 is connected to the image memory function 9c of the control device 9.

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