An apparatus capable of detecting a displacement of a probe from a track of a recording medium caused by track fluctuations and vibrations at a recording/reproducing period. For example, a probe is maintained to vibrate at a frequency and in the direction perpendicular to a data train at a amplitude smaller than the width of the data train. In this case, the amplitude of a reproduced signal of the data train changes with the displacement between the probe and the data train. An envelope signal of the reproduced signal is phase detected using as a reference signal the vibration waveform (standard signal) at frequency f.sub.o of the probe, to obtain a signal proportional to the displacement from the data train. Using this signal, it becomes possible to feedback control the probe so as to make it trace the data train.