Abstract: A novel and improved system is provided for detecting the apical constriction in the root canal of a tooth, in which the root canal, from which the dental pulp has been removed, is filled with electrolyte solution, a first electrode is put in contact with the mucous membrane in the oral cavity, a second electrode is put in the root canal, a fixed voltage or current is applied between the first and second electrodes, and a third electrode having an insulating coating except for the tip thereof is inserted into the root canal to detect sudden change in the potential along the root canal, and this change is visually and/or audibly indicated to indicate the apical constriction.