A position detecting system, for detecting the position of a substrate having a surface, with respect to a first direction perpendicular to the surface of the substrate and a second direction perpendicular to the first direction, by use of a mark formed on the substrate and having an optical power, is disclosed. The system includes a directing device for directing a radiation beam to the mark so that the radiation beam is deflected by the mark; a sensor for receiving the radiation beam deflected by the mark, wherein the position of the radiation beam incident on the sensor is shiftable in a third direction in accordance with the position of the substrate with respect to the first direction and also is shiftable in a fourth direction, different from the third direction, in accordance with the position of the substrate with respect to the second direction; and a detecting device for detecting the position of the substrate with respect to the first and second directions on the basis of an output of the sensor corresponding to the position of incidence of the radiation beam upon the sensor.