PURPOSE: To correct the measure to the unequal gradation arising from the nonuniformity in the input and output characteristics of an optical writing type spatial optical modulator which arises without fail in reflection type liquid crystal projector with which supplying of reading out light with perfect parallel light beams is possible by changing the use voltage range (white and black voltages) of a liquid crystal display device.

CONSTITUTION: Linear input video waveforms are inputted to the video signals of a liquid crystal writing device 11 composed of the liquid crystal display device 7, a liquid crystal display device driving circuit 8 and the video signals by a video signal generating section 9 as an input writing means to the optical writing type spatial optical modulator 10. After the use voltage range of the liquid crystal display device 7 is set, correction is executed in the liquid crystal display device driving circuit 8 in such a manner that the output video waveforms of the final stage via the voltage-transmittance characteristic possessed by the liquid crystal display device 7 attains linear characteristic waveforms.