An optical modulation unit is constituted by a light source periodically turned on, and an optical modulation means including an optical modulation element and periodically turned on. The optical modulation unit is driven by changing a voltage applied to the optical modulation element depending on given graduation data so as to modulate an overlapping time between an ON period of the optical modulation means and a lighting period of the light source. The gradation data may be analog gradation data and may be carried by light illuminating the optical modulation element synchronized with a voltage applied to the optical modulation element.