Abstract: In a position detecting method, a light beam is radiated onto an object on which a position detecting mark is formed; the reflected beam is detected by a beam detector through a slit; the slit is vibrated, and at the same time a detection signal outputted from the beam detector is detected synchronously with the vibration of the slit; thereby detecting the position of the object. A fundamental wave component a.sub.f and a second harmonic component a.sub.2f of the detection signal are fetched due to the synchronous detection and these components a.sub.f and a.sub.2f are subjected to the conversions for eliminating the variation in output level in association with the variation in input level while the corresponding relation between the peak point of the fundamental wave component a.sub.f and the zero point of the second harmonic component a.sub.2f is held. For example, as these conversions, the following conversions are performed: ##EQU1## The output components a.sub.f * and a.sub.