A rotatable file dental treating device (1) driven by a motor (2) includes elements (3, 18) for determining a root-canal position instruction related to a tooth apex, elements (4) for stopping the motor according the file (1) position with respect to the position instruction and first members (5, 6) for measuring and calculating the canal length (L) remaining between the file and the position instruction. The device includes second members (7, 7a, 7b) for calculating a value (D) derived from the remaining canal length and third members (8, 8a, 8b, 8c, 8d, 8e) for combining the derived value and the remaining canal length and the combination of the derived value. The canal length produced by the calculating elements (6, 7, 8, 8a, 8b, 8c, 8d, 8e) controls the motor turn-off in such a way that the rotation of the file is stopped in anticipation of the position instruction.