<p>Abstract : Purpose : To achieve satisfactory results after root canal treatment, bacteria and debris present in root canals should be removed. Sometimes, infectious or necrotic pulp tissue present in a lateral root canal causes pain after root canal treatment. Although thorough cleaning of the lateral root canal is required, it is difficult to do. Moreover, it is very difficult for dentists to detect and locate lateral root canals, except in cases where detected incidentally on dental radiographs. We have developed an improved model of electric root canal meter, Justy Ⅲ (m-EMR), which can detect lateral root canals thanks to an improved calculation system. The purpose of this study was to investigate the ability of this prototype to detect lateral root canals by clinical evaluations using dental cone-beam CT (CBCT) as a standard.</p><p> Materials and Methods : With the approval of the Tohoku University Hospital Ethics Committee, we examined 15 patients (2 males, 13 females) who had chronic dental pain after root canal treatment at Tohoku University Hospital. After detecting a lateral root canal using m-EMR, the same tooth was imaged using CBCT. This CBCT image was used to measure the length from the entrance of the lateral root canal to the coronal top of this tooth. Data gathered were statistically analyzed using paired t-tests.</p><p> Results and Conclusion : The mean length using m-EMR was 15.36±3.28 mm, while that using CBCT was 15.65±3.31 mm, showing no significant difference between the two measuring apparatuses. By using m-EMR to detect the presence and location of lateral root canals, it may be possible to determine the location with clinical accuracy comparable to CBCT.</p>