An apex-locating method and device for determining the depth position of the apex in a dental root canal. It uses a device making it possible to form a circuit including a first probe electrode inserted into the root canal of a tooth, a second electrode in conductive contact with an oral mucous membrane, frequency-generating elements able to produce alternating electrical signals at a number of frequencies, and elements for measuring electrical magnitude of alternating signals in the circuit. Provision is made for exciting the circuit and measuring the levels of magnitude of the alternating signals, respectively at low frequency and at high frequency and for detecting a point of intersection where the two levels measured at low and high frequencies meet and become substantially equal, these frequencies being sufficiently far apart for this point of intersection to exist. This point gives the position of the apex.