A device and method for measuring the positional deviation between a plurality of diffraction gratings (2a,2b) formed on the same object (1) include an illumination optical system (3,6,20,21) for illuminating the plurality of diffraction gratings with a light beam, the illumination by the optical system generating a plurality of diffracted light beams from the plurality of diffraction gratings, an interference optical system (22,23,10) for forming at least one interference light beam from the plurality of diffracted light beams, a detector (12,14) for detecting the at least one interference light beam, the result of the detection serving as the basis for measuring the positional deviation between the plurality of diffraction gratings, and a measuring portion (31-33,41-44) for measuring the relative positional relation between the illumination optical system and the plurality of diffraction gratings.