PURPOSE: To measure moving distances continuously with high accuracy and resolution by changing over the periodic outputs of detectors which count according to the facing states of the adjacent detectors and a scale and correcting the phase deviation distances between the detectors before and after changing over.

CONSTITUTION: The period voltage outputs generated in photodetectors 21, 22, 31, 32 by the slits on a scale 1 and the slits of a detecting slit plate 25 are shaped at 6W9 to square waves. The pulse outputs quadrupled by multiplaying parts 11, 13 are transmitted to a changeover part 17. In direction discriminating parts 12, 14, the phases of the outputs of the parts 7 and 9 on the basis of the outputs of the parts 6, 8 are detected and are transmitted to the part 17. The outputs of the parts 6 and 9 are introduced to a changeover output generating part 16, and both these shaped outputs have a 90° phase difference, and the periods for generating the outputs of the detectors 21 and 32 are discriminated by the part 16. The deviations in the disposing time of detectors 2 and 3 are operated 19 and counter 18 by measuring the phase deviation times and period of the outputs of the parts 6 and 8, 7 and 9 between each other.