Capability and performance of the pliers type coupling device for connection of sea eel pots to loop of main line and automatic separating system were investigated at sea as first trial of developing automatic operating system for sea eel pot fishing boat. Obtained main results are as follows. 1. Maximum tensile load of the pliers type coupling device when connection with high twist PP dan-line as a loop of main line was 1603 N, higher than when connection with low twist PP twine 1379 N. 2. A retention ratio of sea eel pots from main line by connecting with coupling device was successful 100% with high performance without any loss in the sea. 3. A ratio of automatic disconnection between sea eel pots using coupling device and loop of main line was perfect 100% without any trouble.