A length measuring device for performing length measurement and an exposure apparatus for performing an exposure operation on a first object having a plurality of alignment patterns thereon, includes an alignment detector for detecting the relative positional relation between the first object and a second object having a plurality of reference alignment patterns used for aligning the first object therewith, and for detecting an alignment condition between the alignment patterns of the first and second objects, a movement device for moving the first and second objects relative to each other, a measurement device for measuring the amount of movement of the movement device, and a length measurement device for performing measurement of the space between the plurality of alignment patterns formed on the first object. The length measurement device performs length measurement on the basis of successive alignment conditions detected by successive alignment condition detection operations of the alignment detector between the plurality of reference alignment patterns on the second object and the plurality of alignment patterns on the first object, as well as the amount of movement of the movement device measured by the measurement device during the successive alignment condition detection operations of the alignment detector.