We make an ultrasonic system as a trial to measure the distance between trawl doors, and carried out a water tank ($24{ imes} 24$m, water depth 1 m) experiment for confirming the practical use of the system in October 1996. This system calculates the distance between the pinger (50 kHz) and the transponder (50 kHz/70 kHz) attached each one on the trawl door by measuring the time-difference of receiving with two channels receiver on the trawler. This paper assums that both the length of the warp from the stern to the trawl door is same. At results the system shows a good relation between the distance and the time-difference of receiving while the location of the pinger is moved in variously in the water tank, and it was found that the method of measuring techniques on the prototype system could be applied to the measurement of the trawl door opening in the field experiment.