PURPOSE: To improve the measuring precision by fixing a scale precursor made of an optically writable substance on a supporter and optically marking on the scale.

CONSTITUTION: A tape-like scale precursor 10 is fixed on a supporter 20, and a write head 26 can laterally movably disposed along the longitudinal direction of a scale under control by a signal processing and control circuit 28. In addition, an interferometer system comprises a laser source 32 and a beam splitter 34, which separates a beam from the laser source 32 into a measuring beam 36 and a reflection beam 38. Two reflection light beams from reflectors 30, 40 interfere with each other, the interference fringe produces while the head 26 moves along the scale, and is counted by an interference fringe detector 42. Next, the circuit 28 switches on and off the laser diode of the head 26 at a proper interval, and thereby graduations are made on the precursor 10 at a constant pitch.