To facilitate a zero adjustment operation for adjusting an output voltage for each frequency at the same voltage in root canal length measurement in which two kinds of voltages different in frequency are applied between a measurement needle inserted in the root canal and an opposite electrode connected to a lip, an impedance in the root canal is measured from the difference in output voltage between the frequencies and the position of root apex is determined.

At least two kinds of frequency signals different in frequency are applied between the measurement needle 1 inserted in the root canal 3 and the opposite electrode connected to the lip 5, a voltage caused by a change in impedance between the measurement needle 1 and the opposite electrode 7 in response to each of the frequencies is detected, the output voltages are each adjusted to the same voltage, a difference in the voltage after adjustment is detected, and the root apex position ap is determined based on the difference in voltage. In adjusting both the output voltages by a switch 39 prior to detection of the difference in voltage, a switch 41 acting in the same way as the switch 39 is provided on a file holder 40. Thus, the zero adjustment operation by an operator is facilitated.