A new pathlength deviation detection technique which is composed of michelson interferometer is described and verified experimentally. The technique uses a sub-threshold biased visible laser diode of 20$mu$m coherence length as a low-coherent light source. And for zeroth-order fringe(which is the largest among fringes) identification we used a piezoelectric transducer with a large modulation smplitude, which enables without the need of constant velocity scanning, to distinguish reflection surfaces separated by more than 10$mu$m with a resolution of less than half-wavelength.