PURPOSE: To filter a higher harmonic wave with changing substantially no phase angle of a formed signal by sifting the order of deflection of a grid line according to the arrangement order.

CONSTITUTION: For suppressing a higher harmonic wave accompanying an arcsine function, order, in which respective scale marks (grid lines) (n) in an aperiodic scanning scale 18 are arranged while shifted by about a small value δn in the measurement direction X to the nominal positions of an attached periodic measuring scale individually, is formed. In this case, if the order is formed while deflection order of grid lines is mixed up, the scanning scale 18 gives signal constituents with comparatively large phase difference as to the adjacent narrow scanning positions, and actually generated disorder has influence on the respective signal constituents with deflected phases uniformly. Therefore, a filter action can be obtained. This turbulence does not shift a phase angle of a formed signal at all.