To integrally provide a mechanism, which pushes down the stopper provided to a measuring needle in a freely slidable manner, to a clip holder and to make it possible to push down the stopper by one finger of the hand, which has the clip holder, not only to simplify the push-down operation of the stopper but also to facilitate root canal length measuring work.

A conductive root canal length measuring needle 1a is clipped to electrically detect that the tip of the root canal length measuring needle reaches the root apex and the push-in depth of the root canal length measuring needle into the root canal 3 at this time is replaced with the stopper 4 provided to the root canal length measuring needle in a freely slidable manner to measure the length of the root canal from the length from the tip of the measuring needle 1a to the stopper 4. A measuring electrode wire having a clip part 6a, which is brought into contact with the measuring needle 1a, provided to the tip part thereof and a measuring needle clip holder cylinder 11, through which the measuring electrode wire is inserted, are provided and the measuring needle clip holder cylinder 11 has the operation lever 30 piercing through the holder cylinder 11 in the up and down directions crossing the holder cylinder 11 at a right angle to move and a stopper push-down member 32 coming into contact with the upper surface of the stopper 4 to downward push down the stopper is provided to the lower end of the operation lever 30.