Abstract: A device usable with a first object and a second object at least one of which is provided with a diffraction grating, for detecting the position of the second object relative to the first object, is disclosed. The device includes a light source for projecting a position detecting beam upon the first object; a beam detecting portion for receiving the position detecting beam after it is directed from the first object and being incident on the second object, the beam detecting portion receiving the position detecting beam to detect the position of the second object relative to the first object; wherein at least one diffraction grating is disposed in the path of the position detecting beam to be received by the beam detecting portion, which diffraction grating is effective to diffract the position detecting beam at least twice and wherein the beam detecting portion is disposed at a site effective not to receive unwanted diffraction light produced from the or at least one diffraction grating.