The on-line monitoring system for single edge crack was developed based on the direct current multi-terminal potential drop method. The multi-terminal potential drop method means that many electrodes for measuring potential difference are distributed on one side of plate-like structure, the several combinations of potential difference ratio give the crack length, the crack position and the side on which the crack exists. Thus the algorythm for detection of single edge crack based on the multi-terminal potential drop method was programmed as an on-line monitoring system. The developed system was verified using single edge cracked specimens made of stainless steel, carbon steel and copper. The verification test results showed that the crack length could be detected with an accuracy of ±1.0mm even for a shallow crack, the crack position between electrodes could be detected within about ±1.0mm. Then the multi-terminal potential drop method system is considered to be very much effective as an online monitoring system.