PROBLEM TO BE SOLVED: To enable the automation of work by a root canal forming device and to improve safety and operability by providing this root canal forming device with an automatic feeder for automatically moving the reference position of a cutting tool for a holder in the axial direction of this cutting tool. SOLUTION: A dentist slightly inserts the front end of a file 2 into the dental crown part 1c of the root canal 1a after setting various conditions by a setting section 14, then fixes a handpiece 4 in this states and treads a food pedal 9. A root canal measuring circuit 13 is then operated. When the insertion of the file 2 into the root canal 1a is checked from the measured impedance response value, a motor 6a for cutting and a motor 7a for feeding start rotating and the file 2 is inserted at a specified speed into the root canal 1a while the file 2 executes cutting. When the measured data of the root canal length attains a set value, the motor 7a for feeding is rotated in a backward direction and the file 2 is returned toward the dental crown part 1c. The cutting operation is stopped as well and the measured data of the root canal length is displayed on a display 15.