PURPOSE: To facilitate magnification and reduction of a drawing pattern by scanning a reference beam separated from a light beam synchronously with an optical beam, making the scanned reference beam incident on a scale part having a photodetection section repeated alternately at a prescribed pitch to generate a reference clock signal.

CONSTITUTION: A light guide 11 is arranged at the rear side of the scale 10 and leads an optical pulse signal obtained through the scale 10 to a photodetector 12. The photodetector 12 generates a modulation clock signal turned on/off at a period equal to the period of blink of the incident optical beam via the light guide 11 and inputs the clock signal to an optical modulator 3. The scale 10 is provided with a shaft 13 extended in parallel with the subscanning direction Y and the shaft 13 is fixed to the output shaft of a step motor 14. In driving the step motor 14 by a prescribed step number, the scale 10 is turned around the motor shaft and is tilted by a prescribed angle to the upper face of a moving stage 7.