The aim of this study was to analyze the reliability of an ultrasonic device(SDM) measuring soft tissue thickness in relation to tooth position, and to find factors which can influence the reliability. The results showed that 1. measurement error was the largest in the maxillary second molar position and the smallest in the mandibular central incisor position. 2. in a box whisker plot, the difference between two measurements was most widely distributed in molar positions. 3. in Pearson correlation analysis, the relationship between two measurements was the highest in the maxillary lateral incisor position and, the lowest in the maxillary second molar, mandibular first and second premolar position. 4. a stepwise multiple regression analysis could explain the difference of two measurements with various independent variables in 29.7% (P<0.0001). Gingival thickness was the only variable influencing the measurement difference in a statistically significant level(P<0.0001). It can be concluded that its high reliability, ease to use and patient comfort justified the application of the SDM in measurement of soft tissue thickness.