The present invention is concerned with point/output characteristics of a light input image having density gradation using a light writing type light valve comprising a combination of a hydrogenated amorphous silicon photoconductor and a ferroelectric liquid crystal. More specifically, the invention relates to a method of expressing continuous gradation using a light writing type ferroelectric liquid crystal light valve that has been difficult to realize. The expression of continuous gradation is made possible by superposing a DC bias voltage of from 0.1 V to 10 V on a rectangular-wave voltage or a bipolar pulse voltage that has theretofore been used as a driving voltage in order to strongly couple together the spontaneous polarization of ferroelectric liquid crystal and the polarity of the DC voltage, and by eliminating the bistability exhibited by the ferroelectric liquid crystal while developing a monostable condition.