The invention relates to an apex-locating method and device for determining the depth position of the apex in a dental root canal. It uses a device that makes it possible to form a circuit comprising a first electrode-probe inserted in the root canal of a tooth, a second electrode in conductive contact with a buccal mucous membrane, frequency generator means capable of producing alternating electrical signals at several frequencies, and means for measuring the electrical magnitude of alternating signals in the circuit. The invention makes provision for exciting the circuit and for measuring the levels of magnitude of the alternating signals (I), respectively, at low frequency (f) and at high frequency (F) and for detecting a crossing point (C1M) where the two levels (If, IF) measured at the low and high frequencies (f, F) meet and become substantially equal, these frequencies (f, F) being far enough apart so that this crossing point (C) exists. This point gives the position (M) of the apex (X).