A position detecting apparatus comprises a light emitter which activates a laser source to emit a light beam so that mode hopping occurs, an interference optical system which directs the light beam from the laser source to a measuring light path on which an object to be measured is located and a reference light path on which a reflection mirror is placed and merges the reflected light beams coming back on the light paths so that the reflected light beams interfere with each other, a light sensor which receives the interference light beam provided by the interference optical system, and a position detector which determines the position of the object of measurement based on the interference signals produced by the light sensor before and after mode hopping caused by the light emitter.